How can I download a new owners Manual for my amp?

I have to assume you're really meaning a tear in the speaker cone. If so, yes you can repair it if it's small and yes, it will affect the sound. Peavey Bandit amps have come with a few different speakers over the life of the name, but all will still be available either online or directly from Peavey's parts department. Here's the link-
http://www.peavey.com/support/contactinfo

COntact Peavey for the manual and suggestions as to setup. Here's their link
Peavey com

yes, if you know what is the amp module check.

check your pickups and the cord going to the guitar..for shorts..

Sounds like a bad power supply filter cap...the buzz you're hearing is the 60 cycles hum that filter caps reduce or (hopefully) eliminate altogether. It may also be related to the tubes but finding the bad one will require some work. Any friends have this same amp? If so, try swapping one tube at a time and see if you get lucky. If this amp is new take it back ASAP as it is a manufacturing defect.

Check speaker impedance or disconnect speaker leads, then use a 9V battery to verify cone movement and if this test is ok, then go inside the amp, if there's no fuses blown check opamps supply voltage which should be -15V at pin-4 and +15V at pin-8, let me know your results to help improvement.

Mouser.com

Most Peavey speakers you will have to just remove the grille in front of the speaker. These usually are held by Velcro like fasteners that you must carefully pry the grille out. That will give you access too the screws holding the speaker rim. Be aware that there are different sizes of speaker rims and mountings for the "same" listed size speaker. Also use the same impedance speaker as the original one or you MAY find you have less power OR blow your amp out.

The amp is doing what is called "motorboating". This is usually caused when the voltages to sensitive preamp stages are inadequately filtered or regulated and the sudden load of the output stages cause a dip in the voltages sent to the preamps which in turn vary the input to the output stages starting the cycle over. This will require electronic troubleshooting and if your not skilled in that, then it is NOT a DIY fix. What you are seeing is NOT normal, there is a defect.

You have a bad ground probably inside the amp , you need a on hands Tec for this one. I bet its not to serious , about $65.00, is it worth it to ya all tho I don't know your area , but its been my exp.this would be a fair price most any where. good luck DFD