Ah, this is always an interesting question. Do they have to adapt the tones to the music? In ancient times, yes. There used to be melodies that you would use as a meter for your poetry. One famous example is the lyrics to the pop song Dan Yuan Ren Chang Jiu (但願人長久), sung by pop artistes like Teresa Teng and Faye Wong, which is actually a poem in the cí style written by Su Dongpo. The poet himself actually based the meter of the poem on another melody, known as Shui Diao Ge Tou (水调歌头, http://en.wikipedia.org/wiki/Shui_diao_ge_tou), which is not the melody of the pop song. Nowadays, these rules are not observed so strictly, so that is why some people say that they can't understand what the singers are singing. This is actually a well-known consequence, and there are mondegreens that arise from the mismatch between the intended tone of the syllable and what the melody forces the singer to produce. Good songwriters will, in any case, work very hard to make sure the tones in the lyrics match the melody of the music.

It depends. As a native speaker, I have to say that it is common that I don't know what a singer is singing when I listen to his / her songs. When I want to understand, then I try to listen to them carefully. If even after several times listening and I still don't get the meaning, then I just take a look at their lyrics. Recommendation? I don't know your taste yet.

Chinese songs are useless for learning the language as the tones are changed for the rthythm thus the word in the song isn't the word in the speech

I think Chinese songs are better for learning pronunciation but not intonation, because if people speak with the same intonation in a song in daily conversations, they probably will be hardly understood.

梦里水乡 -- 江珊 You could have a try. Einfach google. :-)