In theory, 60' is farther than a high impedance signal should be sent over simple shielded cable without some kind of help but in practice, it often works very well. However, burying cable that's not made for direct burial (as in, buried in the soil without some kind of conduit or other protection) isn't a good idea if you want it to last for any length of time and this applies to coax, shielded audio cable, Cat5e or speaker wire. Speaker wire, specifically, can be infiltrated with moisture and if it causes any connection to the outside of the insulation, your amp won't be long for this world.
You called the noise 'buzzing'- does it change when you turn the lights off in the garage? Often, florescent lights will cause buzzing in audio equipment when shielding is inadequate or, in the case of using non-shielded wire for carrying audio signal to an amplifier, non-existent.
If possible, lay conduit in the ground and run your cabling through that to go from the garage to the patio for sending the audio signal. Whether you use shielded audio cable or Cat5e is up to you but either will work well. I have a run of Cat5e going from my receiver's Zone 2 output to both my basement receiver and my garage receiver and there's no apparent loss of fidelity and neither system has any hum or buzzing. The source end has a 3.5mm keystone insert that receives the plug from an RCA-Y cord and the other ends have RCA plugs soldered onto the wire. Mine is running through metal conduit.
