I just heard a story about Rabbi Elyashiv (no reliable source). He was asked if one may hear ‘a cappella‘ music (I don’t know if the question concerned Sefirah, Bein Hametzarim or all year round). Rabbi Elyashiv said: I don’t know what a cappella is.
So they brought in a cassette player to demonstrate. As soon as the Rabbi saw the tape he answered it is forbidden. The questioner protested: the Rabbi hasn’t even heard it yet! But Rabbi Elyashiv’s point was that the device itself qualifies as a musical instrument (“klei zemer”).