No audio

I have had my PBX running FreePBX for over 6 months and have been applying the updates as they come available. My last update was on the 20/03/2014 and before then all was well. Please advise how I can rollback as I did not have any backup. I am now failing to have audio on sip-sip calls from extension to extension. The phones ring on either end but when they are picked up there is no audio. Please assist in what I can check or what could be the problem. Thank you.