I think there are few songs which are not sibilant, and as I said I don't listen to vocal music much (currently starting exploring some rock/metal) so I don't have a big collection to test out. But when I compare the same songs to cheap earbuds it's not apparent, although Momentum's have much cleaner sound that's why I could hear it better. The biggest problem is with games or youtube videos, it's hard to find recording which doesn't have ear-piercing "SSS" sounds.
Momentums shouldn't sound like that so something is amiss. Do you have another pair that you can try? Where did you get your pair?