To send large attachments such as mp3 via your gmail check out "AttactchEmail"
AttachEmail for Android - AndroidZoom.com
I wrote a review about it, along with a few other methods to share files via the droid here:
How to Send Files from Your Android Powered Phone - Associated Content - associatedcontent.com
But attach email is what I found to be the best. If you have any others that are better please share!