On the computer that you have used Soulseek:
- if it runs windows, go to C:\Users\YOURUSERNAME\AppData\Local\SoulseekQt\1
- if it runs linux, go to /home/YOURUSERNAME/.SoulseekQt/1
(im not so sure about macos)
Open one of the files present (it doesn't really matter) with a text editor and look for your soulseek username
After finding it, in the next line or two there should be your password