I've always wanted the voice of the plant and I've used the song "Feed Me". I've tried out twice and got called back but I didn't make it. Should I continue to use it or try a differant song? I know the show and I can't think of a song that would match it.

Keep the song, but start trying to find something similar. If you can create the same intentions and character that the plant is within another song, there is a better chance of the director(s) seeing you as the plant. Then, once you're in your callback, blow their pants off with "feed me" or whatever song they provide for you.

In terms of other songs, take a look at "King Herod's Song" from JCS or maybe "You're the Boss" from Smokey Joe's Cafe. Something that's reminiscent of the plant, but giving you more to work with character wise.