I find that podcasts have very low volume on my nexus one, using the google listen application while using headphones. My headphones have no volume control, and the nexus one is turned up to the max. I don't suppose there's any way to up this more? As it is now, I can't listen to podcasts while I'm on the bus/metro since the ambient noise drowns them out. Maybe I can talk to the city to see if they will lower the volume on their public transit systems!
This website has some audio hacks for some phones (looks like HTC phones). Also it may not work on Android 2.1+. You have to rooted to use this.