Users don't really care when errors occur in the application: certainly not enough to have them appear in their face. Make an icon in the bottom bar and show it whenever an error occurs. Clicking it would still show the error dialog.

The contrast between the gray text and the white background on the homepage is quite poor and difficult or impossible to read for many people with vision impairments. I'd suggest darkening the text significantly.

May be it's finally time to split things in modules and separate files, even the tiny ones. So rewriting it in TypeScript or smth. would be much easier and code will be even easier to parse and test.

I found that mel $ repeat ... does not work (it is type-correct, but it gives stack overflow)

mix $ sco (\ f -> return $ osc $ sig f ) $ mel $ repeat $ mel [ temp 300, rest 1 ]

On reason is implementation (mel uses foldl1, at least the default implementation does, https://github.com/spell-music/temporal-media/blob/master/src/Temporal/Class.hs#L24 ) but if I use

... foldr1 (