Yesterday we saw the opinion of Rabbi Eli'ezer Melamed regarding playing or listening to music during the three weeks. The thrust of his opinion is not all music is the same, and what should be avoided is happy or celebratory music ( see here). The primary source for this criteria is Maimonides (Ta'anit 5:14) that says "...qol shel shir asur lismoach bahem... "...it is forbidden to joy with musical songs". In other words, when it is not intended for joy and happiness, it is not forbidden.
In the same text, Maimonides also said before: "Vekhen gazru shelo lenagen bikhle shir..." "The rabbis forbade playing musical instruments (in remembrance of the destruction of the Bet haMiqdash)". Rabbi Obadya Yosef explains that what makes music a source of happiness is not its content (as rabbi Melamed claims) but the fact that is played (live or electronically) with instruments. This is why Rabbi Yosef would authorize only playing or listening to Jewish songs a cappella, i.e., without instruments, until Rosh Chodesh Ab, even if the music is of a celebratory nature.
This is a good example to see how two or more opinions among rabbis are formed even when both analyze one single source (=Maimonides). In this case, the two rabbis understand differently the nature of 'celebratory music'. For Rabbi Melamed is about the content, for Rabbi Yosef, if it's played with instruments.
In the Mashadi community the custom is to avoid listening to live music from the 17 of Tamuz until Rosh Chodesh Ab. After Rosh Chodesh Ab we should not listen to music even when played over the media, MP3s, Ipods, etc. (Mr Nassim Bassalian explain to me that some families in Teheran were strict with music only during the week of Tish'a beAb, especially when--unlike this year-- Tish'a beAb would fall in the middle of the week).