Here is a solution that uses find, sed, and xargs.

It first gets the files using find. Then, gets the base name of the file using sed. Finally, it does the move to change the extension.

Note the code below is multi-line for clarity. You should probably execute it in one line.

# split into three lines for clarity
# should execute as one line
find . -maxdepth 1 -name '*.mp4'
| sed -r 's/.*\/([^/]+)\.mp4/\1/g'
| xargs -ix mv "x.mp4" "x.audio"

Below is an alternative solution using basename.

It first gets all the files with .mp4 extension, then gets the base name, and finally renames each with the audio extension.

# split into three lines for clarity
# should execute as one line
find . -maxdepth 1 -name '*.mp4' -print0
| xargs -0 -ix basename "x" .mp4
| xargs -ix mv "x.mp4" "x.audio"

Here is a solution that uses find, sed, and xargs. This solution works even if there are spaces in the name.

It first gets the files using find. Then, gets the base name of the file using sed. Finally, it does the move to change the extension.

Note the code below is multi-line for clarity. You should probably execute it in one line.

# split into three lines for clarity
# should execute as one line
find . -maxdepth 1 -name '*.mp4'
| sed -r 's/.*\/([^/]+)\.mp4/\1/g'
| xargs -ix mv "x.mp4" "x.audio"

Below is an alternative solution using basename. This solution may work with names containing spaces (have not tested this one).

It first gets all the files with .mp4 extension, then gets the base name, and finally renames each with the audio extension.

# split into three lines for clarity
# should execute as one line
find . -maxdepth 1 -name '*.mp4' -print0
| xargs -0 -ix basename "x" .mp4
| xargs -ix mv "x.mp4" "x.audio"