Shazam can determine the artist and song after a few seconds. If you feel participants are holding Shazam under the table and cheating than you're probably going to have to resort to hindering this or not have a music round. The solution is more than just keeping the snippet short. With most DJ software you can have the cross fader in the middle and record a mix of 2 songs onto a file. Make one of the songs more dominant in the mix whereby Shazam immediately recognizes that song and stops right there displaying the artist/song. You of course tell your audience the artist of that song recognized by Shazam and ask them to name the artist of the other song. (Or require they name both songs). Below is an example of this where Shazam recognizes Haddaway's "What is Love" but not the other song. This is just one way but regardless you'll have to get some sort of mixing/recording software and experiment.