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?

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