Mine is using interrupts, so 15MHz is almost necessary because of the overhead from them. I've heard your music on soundcloud. What method does your sound routine use?

Video: http://www.youtube.com/watch?v=6pAeWf3NPNU

Audio only: https://mediacru.sh/qSLb0oNtezJo (Click on the cogs to download in FLAC format)

The music and video are not mine; all I take credit for is getting it to play on the calculator. I did not use any existing trackers to create this, instead writing the music and video playback code from scratch. The video runs at 96x64 at 30FPS, while the music plays through 3 square wave channels and one noise channel.