I know it sounds a bit harsh. But I have a proof-text for it ... 1 Chronicles 15:22 has become my new memory verse:
Kenaniah the head Levite was in charge of the singing; that was his responsibility because he was skillful at it. (NIV)
Seems to make sense to me.
Now it's a little bit of a fudge, I know. For starters I don't believe in proof texting like this (we need to look at a theology of music and a theology of church and the gathering). And the NIV is rendering the Hebrew 'to understand' as 'Skilful at' which is fair enough in the context but other versions bring the point out slightly differently (The NRSV lowers the bar a little: "Chenaniah, leader of the Levites in music, was to direct the music, for he understood it." )
However I think we sometimes are too keen to include everybody in serving in the music team. Should someone who is tone deaf be 'allowed' to lead the singing? Well, no, not necessarily.
It might be a good idea to let them, for a range of political and pastoral reasons. Each church needs to work out what level of skill is required for the task at hand. My church may not need the same skilful musicians as a church of 20,000 people broadcast all over the world.
But I don't think we need to see a theological mandate for participation in the church music team by anyone and everyone who wants to 'use their gift'.
There IS a theological reason why everybody should SING together. But to think this means every singer should have a microphone is a confusion. It treats the band as the church, and puts a false wall between the band and the church (as if you are only participating if you are in the band). They are all participating as the one body of Christ (Gal 3:28) even if they don't have a microphone. We are all one, a body of believers without division, and yet we are also differentiated (1 Cor 12:29).
So let those who are skilful at music (or at least 'understand' it!) lead the rest of us in singing joyfully to our God.