I'm gonna be so honest: you have none.
If you read the homepage for this bot, especially the Warning section, you know that this is a bot which is hosted on a personal home machine.
Because of the very nature of this bot, recordings of your transmitted audio are created when the bot is running.
Because this is a hobby project and I'm lazy, these recordings are stored unencrypted and are not automatically pruned.
I can pinky promise you that I won't ever look at these recordings, but at the end of the day, I could and you'd have no way of ever knowing.
In the privacy game, you should always assume worst case scenario. And in the case of this bot, that would be me opening up all the recordings, listening to them, and laughing at them with my friends. Maybe even throw them on Soundcloud or something, idk.
Now, I wouldn't do that. I like to think I'm a trustworthy person. But the fact that I could should scare you. Privacy without proof is worthless, and in this case, you have no proof whatsoever.
Oh yeah, and there's also the standard disclaimer that I get a lot of information about you from Discord: user ID and a ton of other information that can be correlated to that, what voice channels you're in, who you're with, how long you've been there, bunch of other stuff. And for debugging purposes pretty much all of it gets logged. Gotta have something to look at when everything breaks.
Now, there's basically no shot I'm gonna look through hundreds of megabytes of log files and match up all the user IDs, guild IDs, and channel IDs. If it's not a panic backtrace or bright red error message, my eyes are probably gonna gloss right over it. I'm lazy. But in theory, I could. Spooky.
If you have any concerns about privacy whatsoever, you should probably look elsewhere. Might I suggest Craig?