Recommend a few 4-5 inch: Taidian c430T, Kubi H880FHD, Aino V7000HDA, Newman A65HD, Blue Devil T17FHD.
These screens are Tongbao LTPS screen (clear, high viewing Angle, power saving, 800*480), and the price is relatively moderate, the decoding rate is generally around 1280P, basically the full format support.
7-inch: Taiwan Power C700SP, resolution is not high 800*480, but the battery life is strong and cheap, Taobao lowest price 379.
In addition, I have an Aino v8000HDY, also good, resolution 1024*600, decoding ability is very strong, my computer put the card of 1280P movies, use it to put very smooth, click fast forward also do not card, very good, colleagues are very surprised, now the domestic mp4 decoding ability are almost, very strong. It's usually smooth and cheap.
Recommend a 4.8 inch screen the best non-greasy decoding ability of the touch MP4, Aino v6000hdt, Samsung screen. It has been discontinued, but Taobao still has to sell, of course, the price is high.
Finally recommend Aino novo8 tablet computer, resolution 1280*768, advantages: can plug in mobile hard disk, 8 inch screen, tablet games.
Disadvantages: Low battery life, poor screen material, I-screen. Said so much, finally recommend the Cool H880FHD, because he claims to be the world's top configuration, and supports BBE, in line with your requirements for sound quality.