Bad sound from iMac headphone jacks

[Matthew Buckett <buckett@bumph.org>]

I'm running debian woody on a iMac 350 (slot loading) and I seem to get 
problems with the sound when it is sent through the headphones socket on 
the right of the machine. The sound is rough, not crackly, it's 
difficult to describe but definatly wrong. I think the sound is ok 
through the speaker but it is not as easy to tell. I do not have this 
problem under Mac OS9 or Mac OSX. I'm using dmasound_pmac and the 
standard debian kernel:

i2c-core.o: i2c core module
i2c-core.o: adapter uni-n 0 registered as adapter 0.
i2c-core.o: adapter uni-n 1 registered as adapter 1.
Found KeyWest i2c on "uni-n", 2 channels, stepping: 4 bits
i2c-core.o: adapter mac-io 0 registered as adapter 2.
Found KeyWest i2c on "mac-io", 1 channel, stepping: 4 bits
i2c-dev.o: i2c /dev entries driver module
i2c-core.o: driver i2c-dev dummy driver registered.
i2c-dev.o: Registered 'uni-n 0' as minor 0
i2c-dev.o: Registered 'uni-n 1' as minor 1
i2c-dev.o: Registered 'mac-io 0' as minor 2
dmasound_pmac: Awacs/Screamer Codec Mfct: 1 Rev 3
PowerMac Screamer  DMA sound driver rev 016 installed
Core driver edition 01.06 : PowerMac Built-in Sound driver edition 00.07
Write will use    4 fragments of   32768 bytes as default
Read  will use    4 fragments of   32768 bytes as default

Also is it possible to have the speakers muted when headphones are 
plugged in as under Mac OS(9,X)?

Thank you for any help

Matthew Buckett