Enter the following in the registry :

[HKEY_LOCAL_MACHINESOFTWAREMicrosoftMediaPlayerSettingsMP3Encoding]
"LowRate"=dword:0000dac0
"MediumRate"=dword:0000fa00
"MediumHighRate"=dword:0001f400
"HighRate"=dword:0002ee00

This corresponds to 56, 64, 128 and 192 Kbps.

You can change this to your liking using the following dword hex values :

320 Kbps = dword:0004e200
256 Kbps = dword:0003e800
224 Kbps = dword:00036b00
192 Kbps = dword:0002ee00
160 Kbps = dword:00027100
128 Kbps = dword:0001f400
112 Kbps = dword:0001b580
64 Kbps = dword:0000fa00
56 Kbps = dword:0000dac0