Wolph , your example plays just fine, no volume drops what so ever... I tried on Firefox, IE 11 and Chrome - NW.js and got the same results.
So, I can only think of two explanations for what you describe.
1) There is something wrong with your PC.
2) You are experiencing a psychoacoustic phenomenon. Due to the nature of the "ouch" sound (very loud, sharp and short without any tail) the brain "lowers" the volume perception sensitivity, in order to protect us. Since the sound is so short, there isn't enough time for the brain to compensate this initial sensitivity drop, and we experience the background music as being lowered in volume.
Try this: play the "ouch" sound at -60 db. I bet you wont hear any difference on the music's volume. Or try another sound, a more subtle one that has a bit of a falloff, a reverb or something... It will blend much better on top of the music.