Exhaust system lifetime

How long should a muffler/exhaust system last? I may be barking up the wrong tree, but my 95 E320 sounds like it has dropped a couple of octaves
over the past year. I can hear a significant difference in tone between my 99 E320 and the 95. And I have also noticed that highway fuel economy appears to have dropped from about 25/26 mpg to ~21/22mpg. The car just passed its emissions inspection, so I'm assuming that the catalytic converter is still working correctly.
Any ideas?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.