On Thu, 27 Oct 2011 11:12:00 -0700, DaveC wrote:

is it OK to connect 2 computers' sound cards' grounds together?

No. You will certainly create a ground loop which will result in mains
hum.
d

This kinda kills the project, doesn't it? (At least, the straightforward
solution.)

The alternative is...?


Transformers. You can buy decent ones in Maplin for use in car audio.
Capacitors have been suggested, but they don't help. If they are big
enough to pass bass, the ground loop is still there. It doesn't
require a DC connection to function.

d