The ingame sound and music starts to sound distorted when entering a fight. After the fight the issue remains until I restart the game. Anyone has the same problem and/ or a solution?

It's probably a multi-core problem.

Assuming you're on Windows, try running it on only one core, e.g. by:
- Start the game
- Alt-Tab out
- Open Task-Manager (Ctrl-Alt-Del)
- on the Processes tab, find the executable for the game (DOSBox.exe) and right-click it to get the option to set core affinity
- uncheck all but one core
- apply the changes and go back to the game

Maybe it's also a good idea to turn off all tools altering core frequency and/or voltage (Overclocking tools, Cool-and-Quiet, etc.).

Alternatively I'd try different graphic modes.
Run the "Graphic Mode Setup" in the game's folder and try e.g. DDraw and OpenGL.

Thank you for your detailed reply. Unfortunately none of this helps permanently. I'll have to try other things.

If you say it doesn't help permanently, does that mean it got any better or different somehow? Did something change?

Yes, changing the graphic mode did help at first. I fought a couple of battles without encountering that issue again. But after a while the same problem occured again.

It's okay sometimes, and sometimes it starts after one battle after I start the game. It's not reproducible unfortunately.

I also found out: When I switch to the desktop via ALT+TAB to change it to 1 core like you said, the jittering and distortion is extreme. I have to end the game then, because it won't go away, even I switch back to it.

I am really puzzled here.
But I appreciate your help.

For me DeMignon's solution worked - simply edit the game's dosbox config to launch it in a window and you won't get the distrortion for alt-tabbing.

Thanks for the feedback! One thing I forgot to mention is, that setting the core affinity isn't saved. You have to do it every time you start the game.