Have you tried reversing the connections to the Onkyo amplifier and DCM speakers? Have you checked the cables for continuity? Which model of the Onkyo amplifier are you using?
What normally connects to the input of the Onkyo amplifier and DCM speakers? Does that still work ok?
Have you selected the proper input connection of the Onkyo amplifier? The low volume could be as a result of the input selector not being in the right position or the cables not being plugged into the correct connector(s) And what you are hearing could be crosstalk.
Sounds like the signal is not getting thru on let's say the right channel. If the signal is getting thru on the left channel, plugging that cable into the right channel output of the computer will determine if the right channel signal is present at that connector. By the process of elimination, you should be able to determine where the signal(s) is or isn't.
It's possible you have one or more bad cables or at least questionable connections or as I said the selector switch issue.
Rgds
Ed