Personally I think the Northern Viet accent sounds smoother and more elegant than the Southern Viet accent. Southern accent can sound nice but unfortunately majority of Southerners are lazy speakers and thus make the accent sound bad
Hanoi accent (standard for Northern Vietnamese)
These are excerpts of news from VTV channel of Vietnam
http://boomp3.com/m/21d16cdb77a1/excerpt-from-vtv
http://boomp3.com/m/7d092c855cf9/excerpt-from-vtv-news
http://boomp3.com/m/e68daec1a05e/excerpt-from-vtv
http://boomp3.com/m/0adc5a29a9bd/excerpt-from-vtv-news
Weather forecast in Hanoi accent
http://boomp3.com/m/707712e5427c/weather-forecast-from-vtv
conversation in Hanoi accent
http://boomp3.com/m/b4aa4d534105/conversat...in-hanoi-accent
http://youtube.com/watch?v=pwxT5epS_Ec
Songs in Hanoi accent
http://boomp3.com/m/ab59ae4a2181/%C3%81o-t...%B0%C6%A1ng-lai
http://boomp3.com/m/45a5199e1129/tuy%E1%BA...gi%C3%A1ng-sinh
http://boomp3.com/m/98ed975678f5/hoa-tuy%E...tnamese-version
http://boomp3.com/m/786e55681ed7/l%E1%BB%9...B9-hi%E1%BB%81n
Saigon Accent (Standard for Southern Vietnamese)
Excerpts from TVO
http://boomp3.com/m/52328f15125a/saigon-accent-1
http://boomp3.com/m/c0d6e011fe32/saigon-accent-2
http://boomp3.com/m/0ef9208657ba/saigon-accent
Conversation in Saigon Accent
http://youtube.com/watch?v=u3pzoyGOhdg
http://youtube.com/watch?v=Ki_ej2mlWxc
Songs in Saigon accent
http://boomp3.com/m/7753beec4c6e/%C4%90o%E...ng-th%E1%BB%A7y
http://boomp3.com/m/1b0fcea2a311/h%C3%A0nh...A5t-ph%C3%B9-sa
Sorry I can't find one to represent Central Vietnamese. Central Vietnamese sounds a lot different from North and Southern Vietnamese though