Suggestions/Bugs, in order of importance to me:

I don't know that it's fixable, but it appears that seeking in the song causes the times to get off. I think you mentioned this before. You ought to put it in the readme if you can't fix it. Unless it's there and I missed it.

I can time better with buttondown events than buttonup. Can you change that or make it an option?

In the Click-as-you-sing mode, if you click the existing timestamp, it replaces the timestamp. If you click on a line that already has a timestamp somewhere other than the timestamp, it adds an additional one.

Ctrl-Arrows, I think, would be more intuitive for time tweaking than Ctrl-A and Ctrl-B.

You've misspelled "subtract" as "substract" in many places.

I think the edit mode would be easier to read if it were in columns rather than having the timestamp jammed right next to the lyrics.