Here are a series of files to give you some idea of the kinds of comparisons that can be made using the Sonnox Fraunhofer Pro-Codec plug-in. Note that all these files are 16-bit/44.1kHz WAVs bounced out of my DAW system while monitoring the plug-in's output, so you should be able to import and compare them easily within any multitrack studio software.
ProCodec_InputPCM
The source audio for these tests: a section of my June 2011 Mix Rescue remix, at 16-bit/44.1kHz resolution. It's being passed through Sonnox Fraunhofer Pro-Codec with the plug-in's onboard bypass mode ('Input Monitor') enabled.
ProCodec_MP3_CBR_128kbps_HighQuality
The sound of the MP3 codec running at a constant bit rate of 128kbps with the Highest (Slowest) Quality setting.
ProCodec_MP3_CBR_128kbps_HighQuality_Difference
The sound of the Difference signal when running the previous example's MP3 codec.
ProCodec_AAC-LC_CBR_128kbps_HighQuality
The sound of the Low Complexity AAC codec (as used by iTunes) running at a constant bit rate of 128kbps with the Highest (Slowest) Quality setting.
ProCodec_AAC-LC_CBR_128kbps_HighQuality_Difference
The sound of the Difference signal when running the previous example's Low Complexity AAC codec.
ProCodec_HE-AAC_CBR_40kbps_HighQuality
The sound of the High Efficiency AAC codec running at a constant bit rate of 40kbps with the Highest (Slowest) Quality setting.
ProCodec_HE-AAC_CBR_40kbps_HighQuality_Difference
The sound of the Difference signal when running the previous example's High Efficiency AAC codec.
ProCodec_HE-AACv2_CBR_28kbps_HighQuality
The sound of the version-two 'parametric stereo' High Efficiency AAC codec running at a constant bit rate of 28kbps with the Highest (Slowest) Quality setting.
ProCodec_HE-AACv2_CBR_28kbps_HighQuality_Difference
The sound of the Difference signal when running the previous example's version-two 'parametric stereo' version-two 'parametric stereo' High Efficiency AAC codec.