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% cat mycommand
#!/bin/zsh

for str in "starting ..." "running ..." "ERROR: had a problem" "stopping"
do
    if [[ $str = *ERROR:* ]]
    then
        print -r -u2 -- $str            # remove "-u2" to write to stdout
    else
        print -r -- $str                # Add "-u2" to write to stderr
    fi
    sleep 2
done

• regardless of modifying mycommand to send output to stdout, stderr or a mix of stdout/stderr ...
• in my system I hear ERROR <pause> had a problem over my speakers as the line ERROR: had a problem was printed to the terminal
• tee /dev/tty - insures all data coming in on stdout is copied to the terminal at the same time as being piped to grep
• 2>&1 - insures all data coming in on stderr is redirected to stdout (which means stderr is also routed through the tee /dev/tty | grep ... | espeak pipeline)
• grep is used to extract just the lines we wish to pass to espeak; if looking for multiple strings, you can use grep -e '^string1' -e '^string2' or switch to extended regexp which has an alternation operator (grep -E '^(string1|string2...)').
• --line-buffered is a GNU grep extension that insures we hear the espeak output at the same time as we see the ERROR: line
• OP can install espeak via sudo apt install espeak
• OP can replace espeak with whatever sound producing program they wish

This solution (above) will speak the entire line when said line starts with the string ERROR:.

Speaking just the word error or warning for each occurrence of a case insensitive match on the strings ERROR and/or Warning in the line:

./mycommand 1> >(tee /dev/tty | grep -oi -E --line-buffered 'ERROR|WARNING' | espeak) 2>&1

% cat mycommand
#!/bin/zsh

for str in "starting ..." "running ..." "ERROR: had a problem" "stopping"
do
    if [[ "${str}" =~ ERROR: ]]
    then
        echo "${str}" >&2               # remove ">&2" to write to stdout
    else
        echo "${str}"                   # append ">&2" to write to stderr
    fi
    sleep 2
done

• regardless of modifying mycommand to send output to stdout, stderr or a mix of stdout/stderr ...
• in my system I hear ERROR <pause> had a problem over my speakers as the line ERROR: had a problem was printed to the terminal
• tee /dev/tty - insures all data coming in on stdout is copied to the terminal before being piped to grep
• 2>&1 - insures all data coming in on stderr is redirected to stdout (which means stderr is also routed through the tee /dev/tty | grep ... | espeak pipeline)
• grep is used to extract just the lines we wish to pass to espeak; if looking for multiple strings you'll likely need to enable extended regex support (eg, grep -E ...)
• --line-buffered is a GNU grep extension that insures we hear the espeak output at the same time as we see the ERROR: line
• OP can install espeak via sudo apt install espeak
• OP can replace espeak with whatever sound producing program they wish

This solution (above) will speak the entire line when said line contains the string ^ERROR:.

Speaking just the word error or warning when there's a case insensitive match on the strings ERROR and/or Warning in the line:

./mycommand 1> >(tee /dev/tty | grep -oi -E --line-buffered '(ERROR|WARNING)' | espeak) 2>&1

% cat mycommand
#!/bin/zsh

for str in "starting ..." "running ..." "ERROR: had a problem" "stopping"
do
    if [[ "${str}" =~ ERROR: ]]
    then
        echo "${str}" >&2               # remove ">&2" to write to stdout
    else
        echo "${str}"                   # append ">&2" to write to stderr
    fi
    sleep 2
done

• regardless of modifying mycommand to send output to stdout, stderr or a mix of stdout/stderr ...
• in my system I hear ERROR <pause> had a problem over my speakers as the line ERROR: had a problem was printed to the terminal
• tee /dev/tty - insures all data coming in on stdout is copied to the terminal before being piped to grep
• 2>&1 - insures all data coming in on stderr is redirected to stdout (which means stderr is also routed through the tee /dev/tty | grep ... | espeak pipeline)
• grep is used to extract just the lines we wish to pass to espeak; if looking for multiple strings you'll likely need to enable extended regex support (eg, grep -E ...)
• --line-buffered is a GNU grep extension that insures we hear the espeak output at the same time as we see the ERROR: line
• OP can install espeak via sudo apt install espeak
• OP can replace espeak with whatever sound producing program they wish

This solution (above) will speak the entire line when said line contains the string ^ERROR:.

Speaking just the word error or warning when there's a case insensitive match on the strings ERROR and/or Warning in the line:

./mycommand 1> >(tee /dev/tty | grep -oi -E --line-buffered '(ERROR|WARNING)' | espeak) 2>&1

% cat mycommand
#!/bin/zsh

for str in "starting ..." "running ..." "ERROR: had a problem" "stopping"
do
    if [[ $str = *ERROR:* ]]
    then
        print -r -u2 -- $str            # remove "-u2" to write to stdout
    else
        print -r -- $str                # Add "-u2" to write to stderr
    fi
    sleep 2
done

• regardless of modifying mycommand to send output to stdout, stderr or a mix of stdout/stderr ...
• in my system I hear ERROR <pause> had a problem over my speakers as the line ERROR: had a problem was printed to the terminal
• tee /dev/tty - insures all data coming in on stdout is copied to the terminal at the same time as being piped to grep
• 2>&1 - insures all data coming in on stderr is redirected to stdout (which means stderr is also routed through the tee /dev/tty | grep ... | espeak pipeline)
• grep is used to extract just the lines we wish to pass to espeak; if looking for multiple strings, you can use grep -e '^string1' -e '^string2' or switch to extended regexp which has an alternation operator (grep -E '^(string1|string2...)').
• --line-buffered is a GNU grep extension that insures we hear the espeak output at the same time as we see the ERROR: line
• OP can install espeak via sudo apt install espeak
• OP can replace espeak with whatever sound producing program they wish

This solution (above) will speak the entire line when said line starts with the string ERROR:.

Speaking just the word error or warning for each occurrence of a case insensitive match on the strings ERROR and/or Warning in the line:

./mycommand 1> >(tee /dev/tty | grep -oi -E --line-buffered 'ERROR|WARNING' | espeak) 2>&1

The following sends Danger Will Ronbinson to the speaker (max of once per input line) whenever there's a case-insensitive match on the strings ERROR or warning:

The following sends Danger Will Robinson to the speaker (max of once per input line) whenever there's a case-insensitive match on the strings ERROR or warning: