ticket #9248 describes a bug commonly seen in all OLPC deployments: after some time, sound support will be lost in all Sugar activities. Their output is simply silent. No error messages are logged. When this happens, sound from Open Firmware is still perfectly functional: you can still hear the bootup sound (presuming that the user did not mute it by pressing the volume-down key during boot), and sound playback works fine during the Open Firmware rocker-left self tests.
This is due to a bug in the OLPC operating system. As of OLPC OS v8.2.1 (latest official release at time of writing), this bug is still unfixed.
It is not exactly known which activities or components cause this loss of sound to happen. Suspects are Speak, Measure, and Adobe Flash.
When this issue is encountered, you can restore sound by running the following command in the terminal activity:
amixer sset PCM 90% unmute
However, running syntax-sensitive commands in the Terminal is not always appropriate (e.g. for children or non-technical support staff). So I have produced an activity to do this for you.
To install the activity, click this link: http://dev.laptop.org/~dsd/FixSound/FixSound-1.xo
Then you will see the "Fix sound" activity added to the home screen. It has a Speaker icon. Simply run this activity every time you experience a loss of sound - it will apply the workaround, and then exit after 5 seconds.