audio: reduce default ring buffer size

Current size was defined before the LibAV jitter buffer
was introduced. The buffer size defined was already very
conservative: 32000 samples means 2/3 of a second (an eternity).

* reduce buffer size to 16000 samples (333 ms at 48kHz)
* reduces audio latency of up to 333 ms
* should slightly improve echo cancelling

Tuleap: #120
Change-Id: I0c3d4f83d18f4fdede7e89cd9a36327af716c1ca