How do I preview a wav file in the new outlook without having to download the file?

I was also having this issue and found that the application for .wav had been changed to "Media Player".

(Note in the screenshot below that .wax files show "Windows Media Player")

Steps to resolve:

1. Open Default Apps
2. Choose: "Choose Default apps by file type"
3. Scroll down to .wav
4. Change the .wav application to "Windows Media Player"
5. Close the Default Apps window
6. Close Outlook
7. Relaunched Outlook
8. Test

I have regained my ability to preview wav files.

For the System Administrators out there, you may be able to implement a login script that sets this using the following (untested) command:
ftype .wav=WMP11.AssocFile.WAV %1

or the full path:

ftype .wav="C:\Program Files (x86)\Windows Media Player\wmplayer.exe" %1

ftype documentation: https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/ftype

This is enough for all of our managed service customers who use wav file voicemail attachment not to adopt the "new" Outlook among a few other reasons but this is a BIG one and needs to be resolved ASAP

Such an inconvenience to HAVE to download and not just play the message.

How did you get Previewer to come up in the new outlook, cannot find that and/or add-ins either. Thank you

I did some testing. On Outlook (new) we're able to preview .mp3 files just fine. I then took that exact same file, converted into a .wav file and now I cannot preview it. I took screenshots of both. I also went into the default app settings on my Windows 11 machine and both apps are set to Windows Media Player. Is there anyway to add the preview option based on file type for Outlook (new)?