I have a Nook, and here's what I can tell you about trying to use Nooks for this kind of thing:

First, Nooks do support MP3 audio files (and some other audio formats); I presume Kindle does, too. They need to be able to render audiobooks. To hear the sound well, though, you'll probably need an earphone, and expect your battery to drain rather quickly.

The file management is very flat (non-hierarchical) and clubby; if you have a fair number of files, you'll waste lots of time trying to switch between them. It's such a hassle that, rather than exploit the huge storage capacity, I now only put a handful of books and files on the device at a time. The "shelving" capability doesn't help matters greatly.

On the basic Nook, you can only read EPUB, TXT and PDF files. With the higher-priced, Color 3G version you have more format options; the formats are listed on their web pages. Text in the TXT files wraps, so the lines of verses all run together. And I found the text so large (in contrast to normal reading on the device) that very little of the text fits on a single screen.

All accented characters are replaced by question marks. This, to me, is a serious drawback, and prevents me from reading many things I would like to--including many songs.

With PDF files where the text is stored as image scans (such as with most free books from Google Books and similar sites), the Nook can only display an entire page--there is no zoom capability (Kindle, I hear, has a limited zoom feature). This usually renders the text too tiny and indistinct for useful reading. I seem to recall similar problems with PDF files using descriptive graphics rather than images, so I no longer try using PDF files on the Nook at all. If you download EPUB versions instead, you'll get all the OCR errors, and scores will be rendered as gobbledygook.

In short, the Nook is a bad choice if you want to carry around songs. The interface design is criminally deficient for generalized use. While the Kindle's capabilities sound superior in some respects, I doubt it's greatly better. I suspect that the best solution for portable reading of non-proprietary material is an iPad or similar tablet device, which understands the need for intelligent file management and flexible handling of many reading formats--and where the interface design doesn't focus on pushing proprietary ebooks from a single vendor.