Hello Suu,
If you are hearing out of both the left and right channel with everything else but this isolated .MP3 (L Output 1 + R Output 2) and have confirmed that your VST Connections are set up Properly then possibly there is an issue with the imported file. Try taking the .MP3 file into iTunes or Windows Media Player and run the output through your audio interface to see if the .MP3 sounds the same or different.