If you're looking for a great phone built for music, the Nokia 5310 XpressMusic would definitely be a good choice. It not that smal but is slick, its well designed and color combined, it's feature friendly, and is a good music player.. What I like about this phone is it's headphones port which is built using the standard port for normal headphones and other head set devices (unlike other phones with exclusive sockets built for a certain model only). This allows you to use your favorite headphone, and probably gives yu more options on that department.
Another thing I love about it is it's sim card placement, which is not below tha batteries, making it more accesible without taking of those batteries.
This phone has Music Player with 3 Dedicated Music Keys (play/pause, and navigatin), 30 Mb built in Memory (with MicroSD™ Memory Option), Bluetooth™ Technology, a 2 Megapixel Camera with 4 x Digital Zoom, FM Tuner, 2 inch 16.7 Million Colour QVGA Screen (240 x 320 Pixels) 103.8 x 44.7 x 9.9 mm, and is available in 3 different colors (red, blue, and black) And like most mp3players, this phone plays music without the headset too.