What does "shaky" refer to in this kind of context?

Native speaker’s answer
Rebecca
In this context, "shaky" refers to his vocals being weak or trembly during the song due to feeling nervous. It may not have been as clear or smooth as he would have liked it to be. Ex: Oh, I haven't warmed up my voice yet. So, it may sound a bit shaky. Ex: Can we record once more? My vocals were a bit shaky in the last round. Ex: Her voice was a bit shaky at times. She must be nervous!