What if you install DOSBox separately, then point Heroic to open it along with the game's bundled config file? (Iirc dosbox -conf /path/to/conf-file.conf, though can't test now to be sure)

I’m also struggling to get Earthworm Jim from GOG working on my Steam Deck through heroic, so I’m following your journey with attention.

What’s really annoying in my case is that it worked without installing anything except Heroic at the beginning and now it doesn’t anymore and I don’t know what change is responsibles for the situation..

There are a good amount of forum posts for these situations

https://www.gog.com/forum/general/dosbox_games_from_gog_on_the_steam_deck_through_heroic_games_launcher_issues

Back in like 2008, I wrote a SB16 DOS WAV file player/driver utility, and I made a point to find out the exact BLASTER config for a default DOSBox configuration...

BLASTER=A220 I7 D1 H5 T6

I hope I've been of at least some sort of help, I think that H5 T6 stuff pertains to the MIDI music side of the card/emulation.

Edit: Regarding your initial question regarding missing libraries, I'd try what the other commenter suggested, install the flac and EV libraries, but my setup isn't quite configured like yours so I can't confirm from here.

I’ve done it after some struggles. But if I remember right the key was installing dosbox (staging) and pointing at the exe on heroic rather than leave it to the app to decide. I think. It’s my son’s birthday today so give me a shout and I’ll go through my settings later tonight if needed. But yh I remember the frustration. Also I just remembered there’s a tickbox on the advanced panel about some 3 letter word. Ticking that was also essential matter of fact that might be the only important thing. Sorry for being vague. I’ll get some facts later today

Why do you need it to go through Heroic? I bought games from GoG, downloaded the files, and launched it through DosBox without Heroic and it worked.

Install flac and dev libraries.