Here are the beta music tracks for Age of Empires. They contain a few differences in some tracks (extensions and different instruments), and there is one extra track (track #6) which was not used in the retail version, which you never heard before! Track #1 is not included since it is only blank and is snow/a garbled mess. Credit to Cease.
I'm really thankful that "Rain" didn't end up like what it is in the beta. It's my favourite song from the OST and if it was the same as it was from the beta, I don't think I'd be able to stand to listen to it.
But at least now I understand why it was called "Rain".