The AirPort Express was a unique product since it included an audio jack. Thus, the Express can offer audio output features that aren’t present on the AirPort Extreme. The Airport Express is the only wireless router that I know of to include audio output capabilities. As a replacement, Apple would like you to consider buying one or more HomePods or HomePod minis. If your torn between these two Apple products, check out AppleInsider’s comparison of the HomePod and HomePod Mini. There are also a number of third party Airport Express replacement options.
You want to look for speakers or AV receivers that offer AirPlay compatibility. More specifically, look for devices that support AirPlay 2, which Apple introduced in 2018. One of the key features of AirPlay 2 is the ability to send music from your Mac, iPhone, or iPad to multiple speakers located in different rooms of your home. Through Apple’s Home app you can group speakers together so you can play different music on different speakers in different rooms. This allows AirPlay to behave like a multi-room audio system such as those made popular by Sonos. In fact, Sonos offers a slew of AirPlay 2 speakers including the One SL, the battery operated Move, which is rated for both indoor and outdoor use, and others. To help you pick a suitable AirPlay 2 speaker check out these comparisons and recommendations from TechRadar, iMore and the Sound Guys.
If you’re looking for an AirPlay 2 compatible AV receiver then check out the AV receivers recommendations provided by the NY Times’ tech reviewers at Wirecutter.