Any sound you like!
What's important is that...
You are the sole creator of the sound (i.e. you didn't use any library sounds or content that you don't own the rights to) You are legally allowed to share your sound with others You agree to grant the other participants permission to use your submission in their work
Apart from that, whether you share a forest ambience you recorded near your home or a highly-designed sci-fi weapon sound is up to you!