audio: Support 16bit sound source for sound mixer

We have made the sound engine use 16bit buffer to blend multiple
sound sources before the VCS was started, but haven't made sound
sources themselves.  The sound mixing buffer had a higher priority
because further sound sources could yield very little amplitude at
the listener's position.  But now that we acquired high quality sound
sources, we also want to make buffers for them have higher resolution.

8bit sound sources are still supported, although that backward
compatibility rarely helps anything.

Ogg Vorbis files are always decoded into 16bit buffer, while Microsoft
WAV files reserve their sample bit depth in file format.
The raw WAV file can have 8bit PCM data, which have no benefit to
expand to 16bit.

Stereo sounds are still not permitted.