Transferring Music From iPhone To iTunes?

I have all my music for my iPhone stored on Dropbox because my old computer broke and I need to transfer all those songs to my new computer's iTunes library so I can sync my iPhone with the songs. Now I want to get some more new songs and sync them to my iTunes folder so I'm just wondering what the fastest way of getting the songs from my iPhone to my new computer's iTunes library?

10 points for best answer.

Doesn't work from iPhone back to iTunes. Without help. Hehe. Best way would be to download from Dropbox and add to iTunes. Backup backup backup

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