Well, the UTAU issue won't be a problem since it's yours, but...
There's several layers that you will have to encounter, from the top-down...
1. The original song creator. Whoever they were, they were the original creator, and when it comes to copyright in a majority of places, it means they automatically hold copyright. So you will have to ask them for permission, and maybe pay (aka a license) in order to have the basic rights to publish them on spotify and earn money off it in an official capacity. (Since we're talking spotify. Free, unofficial-type releases such as soundcloud and youtube usually goes, though they still actually hold copyright, it's just that most people don't bother enforcing it on their end if you're not making any [KROMER]])
2. The UST creator. Since your work is based off their hard work porting the song over to the UTAU format, you'll also have to ask those people. You can bypass it, however, by re-creating the covers from scratch, which is the best and ultimate way to do anything.
2.5. Instrumental creator, if you used unofficial instrumentals.
Like, okay, I haven't created anything, but let's create a simulation where I did. I, as the original creator of the song, released it because hard work is hard work.
Life's good, maybe I put it on Bandcamp or work with an indie publisher to slam it everywhere (With permission gained from the voicebank holder, naturally.)
Then, all of a sudden, out of my control, some plonker makes a cover and puts it on Spotify, potentially earning money off what is my hard work creating the lyrics, vocals and instrumentals (which I also released, because making covers is part of the culture).
Okay, time to load my copyright shotgun and go to town! Infringing thing has been taken down, and a bunch of fires flame.
... End simulation.
Restarting...
I, as the original creator of the song, re-
Life's good. Put places, got cash, all in order.
I check my email, and someone requests to release a cover of my work to Spotify.
I call their preference for Spotify cringe, but since they had the courtesy to ask, I now have a choice: Deny them entirely, work out a deal that still nets me some monetary gain for my work, or just let them do whatever.
If I choose to work out a deal, then negotiation starts, and there is a chance both parties come out pleased, or if not that, okay with the result. (Say, for example, the cover gets released under my banner and I get the moni, or the classic pay up-front for license or pay royalties as money gets in, or whatever else.)
In the end, it's all up to what you want with your cover. If it's mostly to release for your own enjoyment, slap it on youtube or soundcloud, and find some way to link it up, or more preferrably, download all your music as local files and play them with your devices media player.
If you do want to release it in an official capacity like with Spotify, then you'll have to do a bunch of negotiating.
When it comes to releasing it as an album, good luck with the other alternatives, but you could do some naming to ease the issues should you go the You/Cloud way.
This has been my intro to a little bit about copyright. Personally, I'd rather just create the songs from scratch.