I wouldn't have used 'by' with 'overcome' in your second example. I think 'overcome with' would be more usual. Nevertheless, this would still sound very strange, as one would be more likely to say they were 'overjoyed'. I'm not really sure whether either 'overwhelmed' or 'overcome' can ever be used in a positive sense. At least, non occur to me at this moment.Overwhelmed with and overcome by usually need to be followed by something that will indicate whether it's used positively or negatively.
I'm overwhelmed with work. (negative)
The workers were overcome by joy after winning the lottery. (positive)
Five miners overcome by gas fumes. (negative)
She's overwhelmed with grief after the loss of her son. (negative)
So in your example, it's still somewhat ambiguous whether the maestro was astonished by her voice because it was very good or because it was terrible. So, adding an extra adjective in there should clear it up. her beautiful voice, etc.