This occurs in the following cases:
SIP devices with buggy or misconfigured VAD stops sending RTP without notifications. After silence sound disappears in the flash-plugin.
Calling to Asterisk IVR: after playing IVR messages Asterisk stops sending RTP and waits DTMF, after receiving DTMF Asterisk continues sending RTP, but the flash plugin does not play anymore.
Workaround: use WebRTC instead.