Thanks, everyone for the suggestions, but I found the problem. In the past, when I saw combined this or that in the puslseaudio volume control, it was going to my HDMI external monitor, causing one hell of an echo. So I didn't choose that in audio options. Then, just by chance, I saw that option as well. The one I'd seen turned out to be THE problem. Once I selected it, lmms had audio again.
As for which lmms, it's the Linux AppImage for version 1.2.2. I thought I'd gotten it from either Alien Bob's stuff or SBO, but I looked in my Downloads/00-done directory, and saw the appimage. And I'm running Slackware 15, not M$ CrashOS.
I finally escaped Crashy and will never go back. An M$ tech "support" idiot told me that when my system would just shutdown without any notice and without my permission, it was NOT a crash if it was M$ doing it to do updates. Right...yeah, sure. Crash.
I'll use wine + yabridge if it helps me get VSTs (by converting DLLs to .so) that I can't find native for Linux, but once I discovered that just as free sometimes means you have to pay up to a few thousand dollars for it, those "50 best FREE VSTs for Linux" (etc) often don't have a single Linux VST in them. Others do. It's all hit and miss. I think sites insert what you're searching for as click-bait or something like that, even if they don't have what you're searching for. I've found some, but the only string instrument I've found (and it's one I definitely want) is a harpsichord. I've found plenty of percussion, including some pianos, drum machines, etc, but no raw drums, hi-hat cymbal, cymbals, vibes (aka vibraharp), etc. So far, that is.
And just for anyone (like me until recently) who might need this bit of information: If you're using a USB <---> MIDI cable:
1) Do not use a USB hub (not even a good one that's powered by an AC adapter); go to a USB port ON your system.
Otherwise, you''ll be going crazy with your MIDI controller suddenly not working, where you have to shut down
LMMS, unplug both ends for at least 20s, then reverse that and hope it works again. Same goes for #2--3 below:
2) Buy one with ferrite chokes on both ends to block random EM noise.
3) Use only the cable length you need to go from your MIDI controller to your computer's USB port (routed so it's
out of the way)
The above list is formatted with indentations, etc. If it loses that, use your imagination---I don't know how to do lists in these forums.... Trivial in TeX, but not in these.
Thanks, everyone!