i tried another one with 256kb/s bitrate. this time everything is ok, i converted it to LP4 (ATRAC3 105K) before i uploaded it to my MD under NETMD MODE.
thanks for your help, guys! u gave me some hints!
the software version is shown as below:
SonicStage : 2.3.00.11120
SonicStage Add-on for Personal Audio Upgrade : 2.3.00.11120
OpenMG Secure Module : 4.0.05.10290
MagicGate Memory Stick Device : 4.0.05.10290
NW-E2, NW-E3, NW-E5 and NW-E8P : 4.0.05.10290
OpenMG CD : 4.0.05.10290
CD Walkman : 4.0.05.10290
Hi-MD : 4.0.05.10290
Music Clip, NW-S4, NW-E7 and NW-E10 : 4.0.05.10290
HDWM : 4.0.05.10290
Net MD : 4.0.05.10290
CD-R (Audio CD/ATRAC CD) : 2.3.00.11120
Px Engine: 1.1.0.367
i tried again, most mp3 with 128 kb/s or 256 kb/s bitrate are convertable, but the sound of mp3 with 128kb/s which are converted to any type *.oma is terrible. the sound of mp3 with 256 kb/s is normal. i think this problem is due to the quality of individual mp3 file.