I was told that R’ Chaim Kanievsky shlita holds acapella music isn’t allowed during sefirah. I was told this by someone very close to him, who personally asked him the question.
During sefirah, there isn’t an issur to listen to music. There is an issur to dance. Since music easily leads to dancing, we don’t listen to music. All the acapella music these days tries to sound like there are instruments, and they can just as easily lead to dancing. Therefore, many poskim hold they’re not allowed.
You can, however, listen to slow music or classical music according to many poskim. in my car, I have a CD with some Beethoven, Mozart, Tchaikovsky, Mussorgsky, and some others which I enjoy listening to. I also have the soundtrack to Disney’s Fantasia, which is various pieces of classical music.