I have a headless debian box with speakers. Suppose I'm at my laptop and I do

$ ssh headless -X

$ audacious

then audacious' window pops up on my laptop's screen. If I open a file then it is played on the speakers connected to headless (as expected). The same behaviour happens with mplayer (i.e. sound is played on the speakers of headless).

However, when I do

$ ssh headless -X

$ firefox

and play a sound on wikipedia or a video on youtube then the sound is played on my laptop's speakers.

This is unexpected and undesirable. How can I get firefox to use headless' speakers just like other programs do?

Both my laptop and headless use alsa (without pulseaudio), both are up-to-date debian wheezy machines.