Apparently what people say that music without instruments, i.e. acapella is allowed during the Sfira, according to a rabbi I spoke to, has no basis.
I'd like to hear if it's true.
[In case this adds to the discussion referenced]
IIRC - The Mishna Berura does not assur music during sefira. This is because he assurs music all year because we don't have the Beis Hamikdash. The minhag is to be meikil but we are machmir during times of aveilus. According to Rabbi Stilerman, some say that it is a chumrah atu dancing. If this is true, any fast-moving acapella does not get around the problem, lechora.
I saw Rav Belsky Zatzal wrote a kol korei on the subject. His two main complaints were: 1. That distorting the voices or sounds should be halachically the same as an instrument. 2. Recording a voice in different keys and then "playing it" like a keyboard in a way that a human voice cannot manage should be halachically considered an instrument. He noted that this says nothing about the hashkafic issues (which, granted, do not apply to all acapella - for that matter, some acapella lacks this instrumentation he complains about).
Shuv matzasi - this and more:
https://matzav.com/p%E2%80%99sak-from-rav-yisroel-belsky-and-rav-shlomo-miller-on-acappella-music-during-sefirah-2/Full disclosure: I very much enjoy acapella music. I just don't listen to it during sefira or the three weeks.