click here for dBPowerAMP. The new version is not free but it's very good at what it does. Earlier versions were free...it occurs to me that you may find an earlier version knocking around if you look hard enough.
I suspect that if you are using M4a (or AAC) encoded files, then you must have an ipod or itunes. i-tunes has it's own converter for various formats and compression rates under the properties tab. hope this is helpful.