Implement S_ClearSoundBuffer in OpenAL sound

Fix for Bug 5418

This fixes stuttering looping sounds and background tracks after map changes. Sources and buffers were not cleared like they are in Base sound. Bug has been present for at least three years.

Example video of the bug is posted here:
http://www.youtube.com/watch?v=rQAu_YpnP2o

The bug was not actually caused by revision 1712 as stated.