Probably is too late to answer this topic, but anyway.
I didn`t came to face with this kind of problems for any Nokia 5310XM.
It must be a bad series of manufacturing. I am sorry to hear that some of users of this phone have problems.
Hope they solve.
P.S. Try Resofting the phone or set it bag to factury settings. Also if the phone has been bought from a dealer that has changed the software, there is the problem!
Hello!
The best way to upgrade your software for your phone is using Nokia PcSuite found on your CD in your phone box or from Nokia website.
After instaling PcSuite just use the program menu to find the "SoftwareUpdate" and run it. Just follow the instructions there!
Hello!
Nokia 5310 Express Music is tested and used for headphones.
On the loudspeackers the sound quality and volume is much lower.
To be noted that Express Music is for headphones!
In other ways, this phone is great! ;)
Unfortunatly there is nothing it can be done for the loudspeackers

Also for best quality for mp3 and other sounds on this phone use Nokia PcSuite Music Transfer, it converts the files into best fit compression for the phone!
Hope this helps!