Let Them Sing It For You is described as 'Text to speech, but with clips from songs' and is an website in the Audio & Music category. There are four alternatives to Let Them Sing It For You, not only websites but also apps for Android, Android Tablet, iPad and iPhone. The best alternative is Yarn.co, which is both free and Open Source. Other great sites and apps similar to Let Them Sing It For You are Clash.me (Free), Bard - Video Assembler (Free) and Melobytes (Free).
Clash.me lets you assemble soundbites from text input by combining multiple sources. Basically, you enter some text and the web app pulls audio and video snippets from popular sources to create an entertaining mashup.