Try downloading one in itunes from the "photoshop TV episodes" series, I know these work OK on my mac, they are big files, 48 meg so take a while to download
There is a volume adjust control in the podcast, in itunes, click on the podcast, then command - i ( or control click>get info) this brings up the info box, go to "options" tab and check where the volume slider is set. That should then work OK