How to rewind/fast-forward audio messages in iOS
I found one way (tested on iOS13). 


Long press on the audio in Messages, a pop up will appear with 3 options: "Copy", "Save" and "More..."
Choose "Copy"
Switch to the Files app and go a folder of your choosing (I chose a folder under "On my iPhone")
Long press and chose "Paste"
Open as audio file, where you can scrub, pause, fast-forward, etc.