What directory if any, on Linux equivalent to Windows .wav, midi etc sound files collection ?
if not any where some of their Linux' download sources and to put it on Linux the best ?
Is /usr/share/sounds what you are looking for?
/usr/share/sounds/alsa/Front_Center.wav
Or according to this page:
https://wiki.archlinux.org/title/MIDI (https://wiki.archlinux.org/title/MIDI)
if you had a soundfont file perhaps the system location would be:
/usr/share/soundfonts/FluidR3_GM.sf2
Possibly a "user" location would be an equivalent dir under /usr/local/share/ or somewhere in your homedir which you choose yourself.
The xdg-user-dirs package sets up the usual basic user directories in ~/