if it is like my family she may be objecting to bad voices but i suspect she just has sensitive hearing and the combined singing voices were just more than she could handle. i don't think it will always be like that for future birthday celebrations.
is it normal for my 2 yr old to scream and cry when i or anyone sings? . on her second birthday i was holding her and i and everyone else was singing happy birthday and she started crying and screaming and slaped me and said no no stop.