Wirelessly posted (8700g: BlackBerry8800/4.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102)
If the BlackBerry is not working properly, just take it back.
Normally the sounds don't just quit working, and if the Profiles are set correctly, then you have a problem. It is unlikely (though not impossible) two BlackBerrys would have the exact same odd issue--and that you would get both of them.
Charging your BlackBerry has absolutely nothing to with affecting the OS. Charge your BlackBerry to 100% regardless.