well sorry for my misinterpretation, the sound when the volume is 5% is indeed not completely silent. And for that problem above actually happened in some of my unsubmitted Beatmaps. some Beatmaps, for example in the slider case, when the slider head volume is 50% (or even higher) and the slider end is 5%, the slider end still has little sound (not the average 5% that is almost silent)
edit:
okay, i was just doing some experiments with a Beatmap that has the same song type and the result is the same. so, i believe that it's actually depends on the song. might use silent hitsound then