Some sound cards need -5 V to have clear sound or to work at all; when -5 V isn't present the sound is raspy or not working. Why?
If you take any electronic circuit outside the conditions it was designed for, there are going to be consequences.
It's hard to know the exact reason for this particular case without having a schematic to analyze, but one theory could be that the negative rail was used as negative supply for an operational amplifier.
These are often supplied by both positive and negative supplies and the lack of negative swing might in effect clip half the phase and cause severe distortion of the signal.
But it's speculation, really.