An all-too-common problem in recordings is vocal sibilance, typically cause by using the wrong microphone or having it placed too close to the singer's mouth -- we want to hear them sing, not examine their tonsils. And, there are some singers who just naturally have a more sibilant quality in their voice.
This is a very difficult problem to fix once it is in the recording itself, though some audio systems may emphasize it more than others. If you have EQ available, try reducing the level on the 5KHz to 8KHz band and see if that helps.