[Freeswitch-users] recording's channels shifted

Hi Why left and right channels' voice shifted from each other in the recorded mp3? I record the sessions' voice thios way: <action application="set" data="execute_on_answer=record_session $${base_dir}/Record/${strftime(%Y%m%d-%H%M%S)}.mp3"/> When i listen back the mp3, at begining the two chennels are synchonized, but after some minutes there will be a shifting between the channels. Why? thanks, Robert