Quotes are important if there are spaces e used with any data passed.
ulx_playsound sound.mp3 would work
ulx_playsound sound file name with spaces.mp3
might not.
Putting quotes around any console command with spaces is just good practice.
Though, I've not used ulx_playsound.
Megiddo, did you write it to automatically grab up to the .wav/.mp3, so spaces wouldn't be needed? If not, might be a good idea for future versions.
Not so much for user laziness, as just user proofing the command.