Hi,

Since upgrading to R5.5 I have noticed that all audio output sound very low bitrate (difficult to explain how it sounds, but certain sound, like a wistle or other similar hight pitched sounds has a certain amount of digital artifacts like when you use a low bit rate codec)

I have read that there is something called the audio tweaker, but am unsure how it's involved if at all. The issue is as far as I can tell it's on all manner of audio output, be it live TV, recorded playback or Online music streams.
And if I pull the mpg off of the backend and play it on another machine (Windows/VLC) sound plays perfectly.

lspci:
00:1f.5 Multimedia audio controller: Intel Corporation 82801EB/ER (ICH5/ICH5R) AC'97 Audio Controller (rev 02)

Try playing that mpg on your KM box, but with something else
like maybe mplayer.

If it were a recording issue I would mention that higher
numbers are worse sounding when setting up audio in
your record profile. I ran into that myself, jacking up the
slider only to find out something that I already knew...

There is a setting, maybe in mythtv-setup that allows you
to specify the audio bit rate. You might have it set to a value
that is not supported (well) by your hardware. Try other values.

Lastly, you might try swapping in a different audio card for
a test.

Cliff