Unfortunately, We don't have answers to every question. I've been here for 7 years and I still don't get replied to.
I'm not sure how MagicJack works, but is it possible that it mutes all other sounds so you don't have computer sounds playing while talking on the phone? I know for sure that I just ran across an option like that a few days ago. Is it possible that it mutes things and forgets to un-mute them when it's finished?
Another thing is Realtek AC97 sucks. When I was using XP my sound would go out for a few minutes and then it'd play all the sound that happened all at once.
Try uninstalling the ac97 device and re-downloading the drivers off their website.
also, it's silly but I should mention it, go into sound manager and make sure certain sound types haven't been muted. Maybe .wav sounds have been muted but .mp3 hasn't..
I'm sure you've tried using the system without the magicjack or the wireless adaptor?
That's all I can think of at the moment. Maybe someone else with more knowledge of the subject will find this thread and help you out.