Renaming muliple files at once

With a shell loop, removing the shortest "double dot suffix"

for f in *.wav.mp3; do echo mv "$f" "${f%.*.*}.mp3"; done

or (my personal favorite for things like this) with mmv from package mmv

mmv -n '*.wav.mp3' '#1.mp3'

Remove the echo or the -n as appropriate once you are happy that they are doing the right thing.

Read man rename and do something like:

rename 's/.wav.mp3/.mp3/' *.wav.mp3

You may have to sudo apt install rename, first.

In the file browser in Ubuntu, you can select multiple files and rename them according to a pattern by just hitting F2 or right-clicking and selecting Rename.

Here I am replacing x with _by_. In your case you can replace .wav with an empty string.