The HappyFeet app can play 14 different percussion sounds, and in dual voice mode, up to two sounds at the same time. It is important that the volume of the sampled sounds is set to the right level relative to one other. This process is called mixing and is what music producers do when they adjust the levels of different instruments, vocals, etc.
One of the best references we've found to this process is from the Mastering the Mix website.
With these principles in mind, we loaded all 14 sounds into Audacity, and then opened the mixer board (View-> MixerBoard). Then, with the only bass drum sound selected (by clicking the “Solo” button) and the sounds playing in a loop (shift-spacebar), we one-by-one unmuted each sound to see how it sounds with the bass drum. Some were clearly too bossy i.e. they took over so needed to be reduced e.g. snare, hi-hat, tambourine.