I noticed that the audio of games/snes was pretty bad for most games I
tried, so I did some bug hunting and found a few. Patches are attached.

The main issue (patch 1) was incorrect BRR decoding for filter types 2
and 3, which is the sample compression scheme that the SNES audio chip
uses.

I also saw that the noise and echo features of the DSP were
essentially just stubs. Patches 2 and 3 implement noise and echo
respectively.

Finally, I saw that it was using nearest-neighbor upsampling from 32
kHz to 44.1 kHz, which results in noticable aliasing. My first
thought was to pipe the output through audio/pcmconv which has a
high-quality resampler. However, games/snes uses audio write blocking
to control the emulation speed, so using a pipe here was problematic.
It would fill up the pipe buffer, then hang while pcmconv wrote the
upsampled data to /dev/audio, then repeat. Anyone have ideas about a
good way to reuse the pcmconv resampler for this (maybe split off into
a library)? For now, patch 4 changes it to use linear interpolation,
which is a big improvement and may be good enough.

Patch 5 is just a style patch to use the enums already defined instead
of hex constants in a couple places.

Here are some samples demonstrating the specific problems:
- BRR decoding
  https://mforney.org/misc/ct-122a-old.flac
  https://mforney.org/misc/ct-122a-new.flac
- Echo
  https://mforney.org/misc/ct-206-old.flac
  https://mforney.org/misc/ct-206-new.flac
- Noise (supposed to be wave noises)
  https://mforney.org/misc/ct-s02-old.flac
  https://mforney.org/misc/ct-s02-new.flac