As far as copyrights go, you are a co creator of the work.
As far as "proof" goes that you were in fact a creator of the work in case it goes to a judge, well that's something else entirely. Privacy laws and recording laws vary from state to state and it can get complicated what can and can not be done when secretly recording someone. For example, in California, its "illegal" for someone to record someone without their consent. That being said, there is alot of gray areas. Its not like someone can say surveillance cameras are not evidence because they did not seek permission.
IMO, you are being too covert, you'll more likely get your ex-band members to clam up and refuse you the evidence you crave for. Talk to an attorney about admisable evidence, then just talk to your band members with a credible witness to corroborate the dialog.