The Geeteoh board uses the WavTrigger board as its sound source. It has a mini SD card on it full of uncompressed audio tracks. First thing I would do it make a copy of the tracks because it is easy to mess something up. WavTrigger will not work with a SD card larger than 32 GB. You can format larger ones down to 32GB if needed.
When replacing a track, use Audacity to save whatever file you have as an uncompressed WAV file. When you save it will ask you what metadata you want included in the file. You must clear all metadata. The WavTrigger is very fussy about meta data, usually it will refuse to play any of your tracks if even one has some meta data in it. Other times it will play oddly.
The other thing to do is open up some of the various tracks already included using Audacity and observe the waveform size. Make sure your new tracks are similar. Ctrl-A selects the track audio, then go to the Effect tab, and use the Amplify effect. There you increase/decrease the waveform. Then save. This will ensure your tracks are not oddly louder or quieter than the others.
The other thing is you may want to play with the track a bit. Since this is a pinball machine with multiple sound happening, sounds with large variations in volume within the one track may not sound great. You want roughly similar loudness throughout the track. Use the Audacity compressor for this. It is a signal compressor, nothing to do with data compression. Then set use Amplify as the last step, then save the track.