Our congregation has an "unofficial" policy of vetting the lyrics of songs before admitting them for use in services. In general, we look at almost every song, regardless of source, and assess each on its own merits. I thought it would be useful to share the things I think about when I am asked to vet songs.
Why is this important? The main reason is that songs are a powerful teaching tool. Music evokes emotion in people, and so words attached to music in song are more memorable. There is a potential for more people to get their theology out of singing songs than from listening to talks, or doing Bible studies.
Now, to the list:
Is it sound? Clearly if the theology is unsound then we shouldn't sing it.
However, it is a bit more subtle than that. Songs are meant to be sung through, which means that the singer wouldn't have much time to think deeply about the words and whether they are wholly correct within the context. As a result, I tend to frown on lyrics which are theologically ambiguous, or are correct only if you "add an asterisk" and say "terms and conditions apply".
Does it have real content? As I mentioned earlier, songs teach. One wants to have songs that carry real, substantial content, rather than ones that only has emotional fluff.
That does not mean one can't have emotions in songs. Music, by nature, is emotional. But emotions should be grounded on solid truths, rather than the transient dispositions of human beings. The Psalms is an excellent example of emotions grounded in a solid knowledge of God.
A simple test: if you replace the word "Jesus" with "baby" and the song still makes sense, then there not be a lot of content.
Is it understandable? Ideally the language should be understandable to the audience. This is particularly relevant to older songs, which may use language which is not familiar to contemporary audience. While having difficult language is not a "show-stopper", people using them should be aware of the need to explain the language.
That's what I can think of so far. Any thoughts on anything else?