What do you mean no distinction? Are you referring to the fact that music is assur the rest of the year? Because even so there is a stronger issur during the three weeks. you want a mekor?
Kli shir
The aruch hashulchan 493:4 nogea sefira ????? ????? ?????, ?? ?? ???????? ???????. ??? ??? ????? ???? ???? ???. ??? ????? ????? ????, ??? ?????? ??????, ????? ??? ??????? ???????.
All the following poskim all say that recoreded music is no less than a kli shir.
Igros Moshe OC 1:166,3:87, MInchas Yitzchok 1:111, Yechave da’as 3:30, and Igros moshe Yoreh deah 2:137 assures a kli shir even when in the house and is not gorem simchah. (look in the Minchas yitzchok who says minhag tov kneder vassur lifrotz geder)
The shibolei leket 8: 127, Oz nidberu 8:58 And the Tzitz Eliezer 15:33 Ashrei Ha’ish (rav elyashiv) in hilchos Bein hamtzarim and I believe in Halichos shlomo in hilchos sefira all say this includes Acapella.