I just find out about QuasarMX for the N9/N950.

I used Sigma Player before (free version) but it lagged from time to time when doing some other stuff (while the song is running in the background)

QuasarMX is still at beta stage but it works very great for me.
The UI is very slick!

The only feature that I'm missing is the equaliser. But this is coming soon!

Landscape supported

Official QuasarMX Website:
http://katastrophos.net/quasarmx/
new website:
https://www.meteorasoftworks.com

The beta is also available on the OVI Store.

Edit:
Soon available for Windows, OS X, Maemo, Android and Symbian.

Edit 2:
It's now available for Harmattan, Sailfish OS, Symbian, Android, Windows and Linux (Ubuntu)

Please buy the PRO version if you like it to support the developers!