I'm using the app "MyProfiles" and that let me change my email alert sound. There is another thread about some people having problems with the app, but I've had it for about a week and it's been working perfectly for me. It really lets you customize a lot of your notifications and create new ones.
Coming from a BlackBerry, I was having withdrawal problems without my custom alerts/profiles. "MyProfiles" has completely solved that for me. Now if only there was a blinking LED notification...