I think the web is a good way to market all sorts of products to niche markets that would be cost-prohibitive to target otherwise.

Find a special interest group
join their forum, find out about their passion
write songs about that passion
people who identify themselves by their special interest WILL spend money to accessorize their identity.

For example, you could make money all day long selling cat songs on a cat lover forum. Or political songs on a political forum... or backing tracks on a poetry forum

You'll never get rich serving niche markets, but there is a virtually inexhaustable mother lode of small opportunities there.