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The IKnowMyABC activity, it seems, should speak the letter names, but doesn't make any sound. To test that audio was working at all, we launched Speak and that spoke.

quozl says:

The IKnowMyABCs activity uses an old explicit GStreamer command line instead of the new general purpose playbin element.

https://github.com/sugarlabs/iknowmyabcs/blob/master/utils/play_audio.py#L70

I suggest looking at the activity logs. If the "unable to play audio file" message appears, then it is likely that some of the required GStreamer packages are missing.