Do muslims sing in the mosque?

Singing (making a better voice) is obliged while doing a prayer. Outside the prayers, in a mosque it is a shame to sing a music (cultural pressure), while it is not forbidden to sing a music without bad lyrics. It is a pressure because some sects doesn’t accept music (they forbid it) and because lyrics can contain forbidden contents, such as about sex, drugs and alcohol. It is in islam forbidden to sing about doing sex, approach drug usement and approach alcohol usement.

Some music are belly dance music and that is not accepted inside islam.

Instead of music, muslims preffer to sing a nasheed (islamic music). That is allowed and sometimes used after and before prayings to amusement and/or educate the mosque visitors. It is also earning charity (hassanah) for the paradise when you sing a nasheed. Most nasheeds in mosques are without any music instrument.