It also works on android 7. But: changing the extension of an audio file doesn’t mean that you convert it to a different Sound Format. If you recorded a sound with the sound recorder component it will be 3gp format. No matter what extension you give the file. Wav and mp3 are completely different Container.
Your recordings will be saved in a WAVE container (extension: .wav, PCM format). Renaming the extension (to MP3) does not change anything with it, as @Django_s_Android_App already said.