I have donwloaded the UM 2.1 demo and saved the installer file to my Windows desktop. I doubleclick the archive and install UM to the default path (\Program Files…). Nevertheless, the file codec_aac.dll is installed to the Windows desktop directory.
The codec file is downloaded to the install location. After that, the installer copies the file to the UltraMixer-plugin directory. You can delete the codec file from the desktop then.