Well if they were covered by utaites that means the song doesn't belong to them; you'd have to contact the producer / the person who uploaded the original song on NND. Look for instrumental versions that they uploaded themselves; they allow you to use those in covers.
You can talk to (and sing for) chorus or music teachers and find out what your range is :)
^ He says a lot of funny things on his feed (often times TMI), but I generally wouldn't doubt what he says. I remember he had a little Q&A session going on a while ago and his answers (in English) were very straightforward and enlightening!
you're welcome! :)
i've found a page that suggests several solutions (skip to the 3rd paragraph):
if, for some songs, you only want to download the audio, keep an eye out for the "Music download" button that *may* show up when you open the menu on a video.
can you tell me how exactly it doesn't work? for instance, the video/audio just won't load? you can try restarting your computer. it doesn't need anything special as far as i know, you just have to be on a nico video for the add-on icon to show up in your search bar, just in case you didnt already know.
the NicoNico Audio Extractor add-on for Chrome is perfect; it downloads the movie or audio in perfect quality!
I use Nicomimi, and the audio always sounds perfect, so...