It's a fair question and I thought it deserved its own thread.If she was meant to be taken more seriously. Then I wish her Power wasn't some random High Pitched sound but actually using her real Melodic Voice to hypnotize people.
It's kind of a moot point in that the character was written specifically for Joan Collins, who never has been known for her singing ability. Certainly when Stanley Ralph Ross created the character, he knew it was for a non-singer. In fact, in the original script, there was no high-pitched sound. It was explained the Siren's note was so high humans couldn't hear it.
If they had gone with a singer for such a major role, it couldn't have been someone such as Lesley Gore. It would have needed to be a woman who also was at least a semi-established actress. Here are the best possibilities from that era, IMO:
1. Julie London. She was known for her sultry singing and smoky voice, and certainly came across in her live performances like a siren.
2. Barbara McNair. She was an accomplished singer who was doing some TV at the time, including a Hogan's Heroes in Nov. 1967 in which she played a Josephine Baker-like songstress.
3. Tammy Grimes. Would have been more of an eclectic choice than the other two, but she had extensive experience in musicals and actually was the original choice to play Samantha on Bewitched, so she certainly could have played a supernatural-type character. But William Dozier had produced the short-lived Tammy Grimes Show, which lasted only four episodes in 1966. One wonders if they would have wanted to work together again.