after i synced my iphone to computer my songs have poor sound quality

I have a iPhone 4 that has over 400 songs on it that I purchased from the ITunes store. A few weeks ago I connected iPhone to computer to download photos and it synced everything on my phone. Now my iTunes songs sound terrible. They have very low "computer" sound quality. How do I get the good sound quality back? Do I need to delete all songs and re-download from iTunes?