This is an interesting albeit very limited hack for 1st generation Samsung phones using WP7 Root Tools. The trick allows you to use basically any size alert on your phone as opposed to the 39 second/1MB limit and you can use different formats, including .aac , .amr , .m4a , .ac3 , .aiff , .flac , .mka , .mp2 and .ogg format (with only .m4a working for SMS).
Now in order to do this, your Samsung phone needs to have interop-access -- not just unlocked -- in order to run the WP7 Root Tools. In theory, if you could get access to the registry and copy/paste files to the /windows folder, you could do this on any Windows Phone, but these tools are a bit limited now.
Finally, if you set your alerts too long, there's no way to dismiss them. It's a complicated hack and seemingly a lot of work just to have a wicked cool and long alert, but hey, it's your phone. While we want to change our email and SMS alerts like everyone else, we'll wait till Microsoft hopefully offers some more customization in further updates.
Get the instructions from XDA Developers.