Audio with Sophia Rev C

[Doug_in_NC]

I have an 800XL with a Sophia Rev C (ie DVI output) card installed, that gives me a great picture on my old Dell 4:3 LCD monitor, but I'm having problems with the audio. I hooked up a composite + S-video audio cable to the conventional video out port and to a speaker with a built in amp, but all I get is a loud hum. The same with a cable that just has audio out. It isn't the cables or the speaker because I get decent sound (but meh video) with an unmodded 800XL with the same setup and either cable. Does anyone else had a similar problem and found a solution? I should note that the machine is also modded with a U1MB and I bought it in that condition so I have no proof that the audio issue is connected  to the Sophia mod. I guess it could have a bad Pokey but that shouldn't give interference on the audio lines, right? I haven't tried taking the Sophia out yet, as I was hoping for some advice from someone who has seen this problem already before I start playing swap-a-chip.