I have no evidence that this bug is the same as already reported, so
filing a new bug.

So, I managed to hang audacity. I recorded a sound using microphone and then
tried to play it -- and it hung (without playing anything).

If I kill audacity, then run it again, recover the project and play it
again -- it hangs again.

strace indicates that it is hanging on some mutex:
Process 16893 attached - interrupt to quit
futex(0xa9f3ad0, FUTEX_WAIT_PRIVATE, 2, NULL

I have prepared a core dump of the process, you can get it here:
http://ro-che.info/tmp/audacity-core.gz