Do you mean low volume or low pitch?
I don't know much about music, but I think some songs have a wide range of notes so people sing at a lower-than-normal pitch, in order to be able to sing the high notes more easily.
Another possibility is: since you say you were watching it online, maybe it's just that the audio is of poor quality? This could be due to many things--cheap microphone, bad placement of the microphone, insufficient sampling rate or bit depth, incorrect EQ settings, etc., also, perhaps bad speakers or headphones on your side. This could all affect pitch as well as volume.