Getting back to the topic of kol isha (As I have already posted, no one today knows how to give rebuke. The Chofetz Chaim said that ????? is related to ?????. You have to prove it to him – in a way that he can accept) , there are several reasons to be meikal:
1. They do not have a choice in order to do there military service, which is a great mitzva. Merely being in a place where there is immodesty is muttar for any need (Baba Batra 57b). Rav Aviner posited that an enjoyment one is forced to get is not called an enjoyment.
2. According to many poskim issurei d’rabbanan are allowed ???? ???? (see Responsa Chatam Sofer Vol 2 Yoreh Deah 131 and Mishna Berura 330,8). This, of course, is assuming that kol isha is rabbinic – and there are such sheetot.
3. There are heterim for songs which are not intended for enjoyment (Sedeh Chemed Klalaim Maarechet HaKuf 42 in the name of Divrei Chefetz) and Seridei Esh 2:14. This would certainly apply to the types of songs sung at military ceremonies.
4. There is heter for several people singing together based on ??? ???? ?? ????? (Chatan Sofer Orech Chaim Shaar HaTaharot Yadayim 14).
5. Embarassing the singer in public where the soldier walks out (Rav Carmel).
Thus a rav can certainly allow soldiers to go to such ceremonies. The one to decide is the mara d’atra, the IDF Chief Rabbi (as ruled Rav Yaakov Ariel).
For a general discussion of the issue of kol isha see Contemporary Halachic Problems col. 2 pg. 147.