Note if you have two sounds both normalised to 0 dBFS and play them both simultaneously, there is a high chance of clipping distortion, and the chance increases the more sounds you layer on. This is a normal result in digital audio. Some systems have built in compressors or limiters to avoid this, but if not you can avoid it by playing back (or mastering sounds at) a lower volume, e.g. -6 dB.