Did you receive any messages about surround sound not supported on TV or any other message? I plugged my new Roku streaming stick into an older Philips HD TV and experienced audio loss only when using the Tablo app. Come to find out the older TV did not support the newer surround sound. Ended up replacing the TV to fix the issue.
I am not sure about the express model and what it supports. However, older TV models may not support all the features required to run a modern Roku. Maybe check out the specifications of each to see if there are incompatibilities.