When first starting the Mixxx software it will require the lame_enc.dll in order to properly stream and also record the stream if you wish.
Please note that the lame_enc.dll does not come with the software by default, it must be downloaded, to do so, just follow these steps:
1. Download the lame_enc.dll from here.
2. copy the lame_enc.dll file into the install directory of the Mixxx software and restart the software.
(by default - C:\Program Files (x86)\Mixxx)