This mod was made by ack7139, the former author of thprac. It basically scales your damage in Touhou 17 based on how well you are singing the soundtrack.

To install, simply drop the SingTouhou.exe file into the game folder next to th17.exe, and run SingTouhou.exe.

Sample gameplay: Hard, Extra
How it works is the bars on the right represent different instruments in the soundtrack you can sing with. Choose one and try to follow it.
The bars shrink with time to show you how much time is left in the current musical phrase.
Each bar has two numbers, the left is your accuracy, and the right is the required accuracy for full damage. The proportion of the two is your damage multiplier (approximately, details unknown).
Damage scaling also applies to bombs, so you better get singing or you're not going to clear.
Sometimes in filler sections in the track, no singing is required and the bar will say "Standing by". You'll have full damage in that case.

Headphones highly recommended so your audio output doesn't interfere with the mic input.
Be careful and take breaks if necessary if playing this, it can make you lightheaded.