It may not be common mode current Mike. (current coming back down the coax)
Is is an internal speaker or external speakers on your PC?
If it is external it may be the speaker wire receiving the radiated signal from the antenna. Some clip on chokes (toroids) may cure the problem.
If not then try about 6-8 six inch coils of coax 3 or four feet from the rig.
73 for now,
QRPME