Castlevania

Adding to the comments to get Adlib sound, exit the game in online emulators with Control + Q, so you can run again CASTLE ADL
(ADL needs to be uppercase)

Use command line argument 'ADL' to enable Adlib sounds. Ctrl+B toggles Background Music and Ctrl+E toggles Sound Effects.

You can get Adlib music to work in this game. It seems, by all the youtube videos I've seen, that most people don't know this.
It doesn't surprise me, because I haven't found any "official" way to activate it. I suspect it might be a commandline switch, or perhaps you need to generate a config file of some kind... not sure.
But there is an easy fix: just rename to AD.DRV file to PC.DRV (make a backup first of course). Voila. It certainly less excrutiating than PC speaker music.
There does not seem to be any sound effects though, but that might just be a by-product of this kludge.

Also, like so many rushed PC ports of the time, the interface and controls are a bit baffling. I've worked out so far that ctrl plus certain keys do things like save, quit and turn things like music and sound on and off. It took me a little while to find out how to fire the secondary weapon, but it's up+space. Seems to be a bit dicey though, but that may have to do with the dosbox settings I was using.