like the original poster your speaker is blown. its actually a quick fix for a sprint repair store. i just had mine replaced yesterday. took about a half hour. times will obviously vary depending on how busy they are. having TEP helps quite a bit.
HTC makes great phones but the speakers they use are somewhat cheap. dropping the volume from maxed out down at least one notch will help drastically. figure ive had mine for WELL over a year and my speaker just went out. not bad for running it at max the whole time.