Are you allowed to hear music on Rosh Chodesh sivan?
This would depend on your minhag with regards to sefira. One who kept from Pesach until Lag Baomer is permitted to hear music after Lag Baomer until Shavuos. One who began from Rosh Chodesh Iyar would keep mihagim of aveilus until the שלשלת ימי הגבלה, the 3 days before Yom Tov. In that case music would be prohibited on Rosh Chodesh. There are those who have a custom to keep from Pesach until Rosh Chodesh Sivan. According to this practice as well, music would be permitted on Rosh Chodesh.