Samsung Galaxy S user here, this exact problem was experienced today on the latest release of Nimbuzz available through Android Market.
On Wifi connection I was able to make a single VOIP call on Skype without any problems approx. 18 hours ago that lasted for 5 minutes (as it was merely a test to see if the new handset could handle it as well as my N900 does). On HSDPA however, the call was placed through Skype VOIP to the same cell number as the call that was successfully placed on Wifi, I could not hear them and they could not hear me.
They called me back on my Skype username for a Skype to Skype call through Nimbuzz, the same issue. No sound, neither of us could hear each other...nothing but dead air.
I'm interested in trying the test version, I have nothing to lose - are you able to send it to me via PM?