The original version of SoundMeter, which was available in the App Store soon after its opening in 2008, has been replaced by new subscription-based and perpetual license apps.
If you purchased the legacy version of SoundMeter, you may continue to use it on iOS 12 or earlier. If you need to re-download it, you may do so for free. Please visit Apple's support site for instructions on how to re-download previously-purchased apps:

Redownload apps from the App Store