Making this post to help people find the solution faster. Bought the game expecting some issues on Arch Linux, needs mono installed, get that and it still won't work. Turns out there is a problem with the Unity library and the new version of alsa-1.2.1 (or 1.2.2, won't test) and above . Should work with alsa-1.1.8 and alsa-1.1.9. This is not just a Fell Seals problem, but an issue with a lot or all Unity games on all linux distros with the newest alsa versions, this needs to get fixed by Unity if I'm reading correctly, not an alsa issue directly.

Links explaining the issue with workarounds:
steamcommunity.com/app/699170/discussions/1/2673382428440933136/
reddit.com/r/linux_gaming/comments/f86rj4/alsalib_122_breaks_all_unity_games/
reddit.com/r/archlinux/comments/f86ri0/alsalib_122_breaks_all_unity_games/

TLDR:

1) Downgrade alsa to 1.1.8 or 1.1.9.
or
2) Use pulseaudio.
or just do this
3) Edit\ .asoundrc in your home folder and add the following to the end of the already existing configuration:

defaults.pcm.surround21.card off
defaults.pcm.surround21.device off
defaults.pcm.surround40.card off
defaults.pcm.surround40.device off
defaults.pcm.surround41.card off
defaults.pcm.surround41.device off
defaults.pcm.surround50.card off
defaults.pcm.surround50.device off
defaults.pcm.surround51.card off
defaults.pcm.surround51.device off
defaults.pcm.surround71.card off
defaults.pcm.surround71.device off

To make this easier to search I'm adding a small part of my output from the terminal in here. Forum character limit :(.

Thread 2 (Thread 0x7fa19da93700 (LWP 1511796)):
#0 0x00007fa19f87d7b4 in do_futex_wait.constprop () at /usr/lib/libpthread.so.0
#1 0x00007fa19f87d8a8 in __new_sem_wait_slow.constprop.0 () at /usr/lib/libpthread.so.0
#2 0x00000000011e9615 in ()
#3 0x00000000011e9669 in ()
#4 0x00000000011ea0fb in ()
#5 0x00000000011e9db4 in ()
#6 0x00000000011e9770 in ()
#7 0x00007fa19f87446f in start_thread () at /usr/lib/libpthread.so.0
#8 0x00007fa19f7a23d3 in clone () at /usr/lib/libc.so.6

Thread 1 (Thread 0x7fa19f6a0740 (LWP 1511795)):
#0 0x00007fa19f76f9af in wait4 () at /usr/lib/libc.so.6
#1 0x00007fa178157323 in () at /home/lebucket/GOG Games/Fell Seal Arbiters Mark/game/Fell Seal_Data/MonoBleedingEdge/x86_64/libmonobdwgc-2.0.so
#2 0x00007fa19f87f800 in <signal handler called> () at /usr/lib/libpthread.so.0
#3 0x00007fa19f6dece5 in raise () at /usr/lib/libc.so.6
#4 0x00007fa19f6c8857 in abort () at /usr/lib/libc.so.6
#5 0x0000000000d2cb66 in ()
#6 0x00007fa1781d08e2 in () at /home/lebucket/GOG Games/Fell Seal Arbiters Mark/game/Fell Seal_Data/MonoBleedingEdge/x86_64/libmonobdwgc-2.0.so
#7 0x00007fa1780ddecc in () at /home/lebucket/GOG Games/Fell Seal Arbiters Mark/game/Fell Seal_Data/MonoBleedingEdge/x86_64/libmonobdwgc-2.0.so
#8 0x00007fa19f87f800 in <signal handler called> () at /usr/lib/libpthread.so.0
#9 0x0000000000cb1808 in ()
#10 0x0000000000a09462 in ()
#11 0x0000000000a0c343 in ()
#12 0x0000000000a0c94f in ()
#13 0x0000000000a0cbc0 in ()
#14 0x0000000000e0c5b9 in ()
#15 0x0000000001793685 in ()
#16 0x00000000017ba8cb in ()
#17 0x00000000017bac1e in ()
#18 0x00000000017bad98 in ()
#19 0x00000000017baf34 in ()
#20 0x00000000017aac08 in ()
#21 0x000000000179ffbe in ()
#22 0x0000000000e1c22f in ()
#23 0x0000000000e1c746 in ()
#24 0x0000000000e1d3e7 in ()
#25 0x0000000000d7a361 in ()
#26 0x0000000000d8287a in ()
#27 0x0000000000d8776c in ()
#28 0x0000000000d878f5 in ()
#29 0x0000000000a1ee6e in ()
#30 0x0000000000c4ee4b in ()
#31 0x000000000044dace in ()
#32 0x00007fa19f6ca023 in __libc_start_main () at /usr/lib/libc.so.6
#33 0x000000000045a5a1 in ()
#34 0x00007ffe3ca18a58 in ()
#35 0x000000000000001c in ()
#36 0x0000000000000001 in ()
#37 0x00007ffe3ca19575 in ()
#38 0x0000000000000000 in ()
[Inferior 1 (process 1511795) detached]

=================================================================
Got a SIGABRT while executing native code. This usually indicates
a fatal error in the mono runtime or one of the native libraries
used by your application.
=================================================================

start.sh: line 16: 1511795 Aborted (core dumped) ./"Fell Seal.x86_64"

alsa-lib-1.2.2 kills 90%+ of Linux Unity3d games. There are workarounds; but overall the alsa team's handling of this issue hasn't exactly been great, in my opinion. I haven't checked since I was berated for not knowing that the documented bug reporting procedure was crap, but I think they've fixed it (but have they released the fix? probably not). There are also listed workarounds, some of which actually seem to work for most games.