I have the same problem. I tried instructions above, but those didn't help.
My Sonic Stage freezes when I'm trying to send mp3 files to my MZ-NH700. I also have NET-MD707 and the same problem. Omgjbox.exe uses all cpu time.
When I installed from the CD I did get with my MZ-NH700, the Simple Burner worked when I transfered music straight from the music CD, but Sonic Stage freezed with mp3s.
I hope somebody can help with this problem.
I have Windows XP Pro SP2 with all available updates.
Also next component are installed with Sonic Stage:
SonicStage : 2.2.00.09020
SonicStage Add-on for Connect : 2.2.00.08230
OpenMG Secure Module : 4.0.00.05270
MagicGate Memory Stick Device : 4.0.00.06170
NW-E2, NW-E3, NW-E5 and NW-E8P : 4.0.00.06170
OpenMG CD : 4.0.00.06170
CD Walkman : 4.0.00.06170
Hi-MD : 4.0.00.06170
Music Clip, NW-S4, NW-E7 and NW-E10 : 4.0.00.06170
HDWM : 4.0.00.06170
Net MD : 4.0.00.06170
EMD Plug-in: 1.1.0.84
Playback Plug-in : 1.0.0.53
CD-R Writing Module(ATRAC CD/Audio CD/MP3 CD) : 2.2.00.08230
Px Engine: 1.6.14.507