Hi,
First, what brand muffler, and which model? What diameter exhaust and tailpipes?
Do you know how far back the muffler was mounted in relation to the cab?
Ok, the mounting location is good. It would help to know the manufacturer and exact model number to know if noise you are getting is considered normal for it or not. For example, a Flowmaster 40 series is louder than a Flowmaster 50 series.
Before using anything to wrap the muffler, first make sure that all exhaust hangers are in place and secure. A loose hanger or one that is broken will significantly increase the noise level of the muffler as heard and felt in the cab.
If no problems are found, it is ok to use a high heat wrap to quieten it down. It wont affect performance or cause any problems with the muffler itself.
The optimum solution would be to go with a different muffler, but without knowing the manufacturer or model number, it is impossible for me to say what step up you would need to go with.
Automotive Repair Shop Manager
Automotive Repair Shop Manager, Technician/JA Mentor