Why would they say something that something assur is assur. I guess it is like the poskim who assur smoking all yeaR and also discuss whether it is muttar on yom tov. Different Dinim.
That makes no sense. There’s no need for an extra stringency when it adds nothing to the original issur.
There is no makor that there is an extra gezeira on music in sefira than the rest of the year. Even the Magen Avraham is only giving a limitation on se’udos reshus – saying not to have dancing at these parties.
If you want to find issurim you’ll find them. But bederech klal if something is not mentioned in the rishonim and no one even talks about it until pretty recently and doesn’t have any clear rayos, you don’t have much of a tayna on someone who dismisses it.