Go to http://lds.org/music/ Click on "Library". Choose a song you want to sing. Click on the play button that will show up as you mouse over the song to hear the MP3, OR, click the title and see the totally awesome music player. It will play your music at the tempo you choose and even transpose the music into a key you can sing. It's FANTASTIC. We use it every day in our Seminary class. I put it up on the TV screen and we all follow along as the words scroll.
This is great resource for when you need to use music in your lesson but don't have access to a pianist.