First: I'm trying to get some old windows games running in wine that use midi files. I've installed a soundfont from slackbuilds.com, and confirmed that midi playing works on the slackware side, but I need to know how I can run fluidsynth as a daemon so I can use midi in wine.

Second: a dumb question that is related to this midi issue. WineHQ's midi page listed registry entries that simply don't exist on my system because a midi device is not configured at all. Would rebuilding wine fix this issue without any need to untangle my wine registry? Or is there a more elegant solution to this?

Hmmm. old games are 100% 32bit, so I would approach that with a 32bit os and 32bit wine, if you have a choice. Otherwise, on a 64bit linux you may run into library compatibility issues big time.

I have a Multilib system, installed fluidsynth as per instructions. The sound worked, but the games sucked, so it didn't last long. It was a windows 98/xp thing.

Maybe you're overthinking it? Fix the errors until it stops complaining.

multi lib 32bit from alien bob is all thats needed to play old games. I do it on slackware current build 18 and debian 10.7 buster.
you need to setup wine to use midi files.

Daemons? We don' need no steenking daemons! (Here's hoping you're a Bogey fan and get the joke)

Just an FTR but I'm running Multilib and play several old games on Slackware, like "Deus Ex" initially the retail version and couple years later the GOTY version. Everything worked fine but obviously the GOTY version fits wide screen at high resolution and get's my vote. I also still play "Road Rash" (1994) every now and then, but the gap keeps getting longer... still love the soundtrack and the feeling of speed on some of those old games. I seem to recall getting "Terminal Velocity" running OK but as much as I liked the game, it's not as fun w/o a joystick and I haven't had one of those in over a decade.

Specific to this thread, I played Road Rash in WINE just 2 weeks ago and it sounds just like I remember it on Win95.

That's right.

I forgot to elaborate; on my system, I have alienbob's multilib installed. I still don't know how else I can get midi set up because wine's wiki basically assumed that the registry entries it needs to support midi actually exist on my system- and they don't.

have you tried winetricks or winecfg and playonlinux may do something not sure.
winetricks you can download windows stuff. playonlinux may do that as well I dont know never toyed with playonlinux much. playonlinux may do more configing and install stuff needed for midi per game requirements.

Look, lawnm0wer, there's stuff out there on this. It's not straightforward, because midi was a 'hi-fi' (pause for laughter) sound format from the 1980s, and used in the 1990s also. I got given an .rar file of Space Quest games, found what was out there, got fluidsynth compiled and working by installing anything necessary. It was the work of an afternoon. I also had to install scummvm(?) to play the games. They sucked, performance was <50%, the mouse guided the characters very poorly, the level 1 was on a timer and I couldn't pick up what I needed to get through on the 1st one.

That was a few updates ago. I think I was on wine-4.17, or maybe the one before that - 4.01? I had the relevant stuff and didn't have to hack registers. I always use Alien Bob's multilib. Here we're 6-7 posts in, you still haven't got it that you don't say "I can't" or "it failed" but you lay out the exact error and circumstances.

Get searching online for something methodical, not necessarily the first hit. Get sample midi files and get them playing. Then go at your game.