voice modem problem

Hello everyone,
I'm having voice modem gvc 33.6kb.But the problem is that I
can't hear any sound except the connectiong sound,when try to run
phone dialer application.I can get dialing message & modem can detect
incoming calls.
Other modem I've found is internal MOtorola 56kPH
data-fax-voice modem.But that doesn't detect incoming calls,neither it
gives sound except connection sound.
I've used good speakers & microphones.Tried by attaching it
with sound card & also with jacks provided with modems.
I could get sound some days ago with motorola modem.But now
no sound.
Thanks in advance for the answer.
Bhumika