It looks like something that after all can/should fix Iron itself?
Comment #35 on issue 225423 by firstname.lastname@example.org
: Missing Listen buttons in Google Translatehttp://code.google.com/p/chromium/issue ... ?id=225423
If you're building your own Chromium please be aware that the default configuration *does not* include MP3, H264, AAC codecs. You must add "ffmpeg_branding=Chrome use_proprietary_codecs=1" to your GYP_DEFINES before building.
Google Translate will only show the sound icon if you have those codecs supported.
As far as I can tell, this is WontFix.