Sound muffled?

[TechSupp]

Not sure if thisis the right forum section but here goes. The problem is not that I can;t get sound buit that it is indifferent. It was reported that a PC was not playing sound through the headphones but when new headphones were put on it was the same although there was sound it was just low but everything was turned up. Logged on as admin and lo and behold the sound on windows start was fine. After a bit of investigation it seems that its only certain sound files, namely espresso video sound and some mp3's (problem is its the ones created with audacity that are our radio show). I will play mp3's taken from a cd at a good volume but when playing other mp3's the sound is muffled and a lower volume. Anyone come across this effect before. This PC is one sent to a company to have imaged on to a new batch of PC's we have just bought and thay updated it apparently with the latest flash etc but also a 6in1 codec pack as I found out. Could this be just a wrong or missing codec and if so which one as not had the chance to test the new pc's yet and hope its not the same on them!
Sorry its a long explanation but just wanted to give the full details to see if anyone can help. Thanks.

[Michael]

I would also look at what drivers they used and if possible, update the drivers as required.