Mary,
FILESIZE:
You have options of reducing filesize by decreasing bitrate, but if you are already using an MP3 file, which to upload here means yes, then you probably don't want to go this route as it degrades sound quality too much.
Other options could include:
1) Finding a site that allows music uploads of higher file size. I think this one, for example, lets you do that, but I haven't tried it:
http://www.3hive.com/
2) VIDEO INSTEAD: You might be able to use MySpace (if you have an account there) and if the file size is over their limit for MP3's you could use the song as a SOUNDTRACK to a video, which allows maybe up to 100MB on MySpace. It doesn't matter whether you have pictures or movies as the "movie part" as long as people can hear the soundtrack. You can use Windows Movie Maker to do this easily. Just open that program and drag your MP3 onto the soundtrack line, and save to a movie. Then upload to MySpace.
3) ONLINE FILE SHARING. Using a free online File Sharing site, to upload your files to. Most of these that I am aware of though, would make someone have to download them to hear the song, but maybe that's not the case? I've always used those services to ALLOW someone to download something that was too large for me to email to them. So maybe they have options NOT to download, while still allowing someone to listen to them?
4) LARGE FILE EMAIL SERVICES: Another option is sites that allow you to EMAIL large files to certain recipients. This may be a good compromise. These sites allow you to email the file to whomever you choose, and yet NOT make it available for ANYONE to download. In this way you could just send to the people on BandMix who wanted to hear it, without having the file wandering cyberspace for anyone to take.
I think this service allows to email up to 100MB:
http://www.yousendit.com
.