Is there a way to find out if any voiceover work is needed?

Playing a lot of custom maps in Warcraft 3 and some in Dota 2, I found out that custom voiceovers are rarely used for the creation of the content. Why is it so? By the way, I am very much willing to give my voice to any kind of custom game. That is, in fact, why I created this topic. Where can I find people and mods that might require some voice acting?