Just a thought, on cards that dont have 5 jack sockets, the Mic and line in probably share a socket with something else.
I think on SB 1024 the rear out socket was line in, so you could have 4 channel and no line in, or 2 channels and a line in.
IF the smaller cards have option for 6 channels with no mic, then maybe the mic shares a socket with the bass, which would explain why it has a DC offset, for the mic to run on.
Most sound cards supply the mic socket with a small voltage for electrec mic elements to work.
Maybe thats what is happening here to mine, when its not playing sound, its enableing the mic in on the bass socket even though it has a seperate dedicated mic socket.
One for the scope/multimeter in the morning.