If you can't get 1 call to sound good you clearly have an environmental problem to solve.
I am not sure where you will ever be advised from us to use ubuntu but I guess anything's possible,
We currently endorse centos5 for older hardware, debian 6 for anything recent from the last 3-4 years and are still vetting centos6
There are not really enough details from this post to diagnose your problem.
I suggest you answer every one of these questions.......
What hardware are you using? (based on the willingness to try so many distros I fear virtualized which is a whole separate can of worms).
What device is the other end of the call? Is it a provider or a IP phone or what?
What format are you recording to?
Do the calls sound unsatisfactory in real time or only the recordings?
Does this same problem only happen in your special conference setup or even on ordinary bridged calls?
In general, digital audio records verbatim to the stream that is being received. If you are using g711and recording pcm wav, there is little reason for quality to be changed from the encoding.
You may want to setup another box on the same network and test calling the milliwatt extension or some other tone and work your way back from there.