I'm not expert, but from the many Japanese songs I've heard it's mainly
- Pronouncing お as "oh" (i.e. slightly diphthongized) especially in songs where half the lyrics are English lol
- Pronouncing を as うぉ. This is probably due more to the fact that it is in the W-line, and old pronunciations stick better in slowly and carefully articulated things like poems and songs.
- A distinction between おう and おお, and separation of いい Again, this is probably due to conservative pronunciation. おう was always a genuine long vowel, while most instances of おお came from おを or おほ. This is most obvious in the word おおきい which is often pronounced おをきひ.
- Not discriminating between different allophones of ん, instead only "n" is used. This is usually because "n" fills a mora better, and Japanese mora, not syllables, are usually matched with the notes.
- Again a mora issue, the っ is pronounced a bit differently. In speech 強がっても would be (tsu-yo-ga-*-te-mo) but in song it would be more like (tsu-yo-ga-att-te-mo), almost as if it were つよがっあっても .
That's all my limited experience has noticed.