You can use any 32 sounds you want, so you are not limited to Star Trek. If you search the web for "Soundboard", you can find several sites that host groups of sound files.
You will most likely have to convert the files you do find to the WAV format as noted in the Prepare Audio Files section.
Some sounds may be Open Source (or Creative Commons unrestricted CC 0) and others may be commercial. If you want to use your soundboard in a commercial way or to record on Youtube, it is best you either use unrestricted sounds or buy a license for the material you use. This is the reason in this tutorial we state the Star Trek sounds may be purchased via CD or download.
If you'd like to create your own custom soundboard, you can use your phone to record audio clips and load those onto your NeoTrellis!
Any recording app will work to gather sound clips, and a portable microphone is nice as well (though not necessary!)