Comment 6 for bug 1530365

If pulse is used as the playback device, repeatedly starting and stopping playback in quick succession may lead to a freeze after which Audacity must be force quit. If the computer is running at high CPU or there are many tracks, a quick hit of Space twice to start and stop, or holding down Play with the mouse, may be enough to cause the freeze.

The same issue can occur if you R and space rapidly and repeatedly to start and stop recordings, or hold down the Record button.

If the device is set to an ALSA/hw choice (so bypassing pulse) these issues do not occur.

Al regards this as most likely a bug in PortAudio's support for libpulse, and/or possibly libasound,

This bug is the cause of bug 133 (moving the transcription slider is not allowed to restart playback on any platform, even though it could be allowed on Windows and Mac).