How do I point Sonos to my music on an Airport Extreme connected hard drive?
A Sonos requires that shared libraries are accessed over CIFS/SMB. From your path it looks like this link is for a share over AFP. Can you find a link for the SMB share from the airport extreme ? Looking at the Airport Extreme docs it seems that if you have an HFS formatted disk mounted then it will share over AFP and SMB 

you could try (just a guess) as a link 


  \MyName's Airport Extreme._smb._tcp.local\MAC FREE\free agent backup\FreeAgent GoFlex


You might also consider removing spaces in the paths as these tend to cause pain if the software is not expecting them