That's going to be a problem, because WAV files are uncompressed raw audio. MP3 is a "compressed" audio format, WAV is not. So you could say that you're going the wrong way.

Finally, I'm going to recommend you stop doing what you're doing. Email servers are clogged enough with all the spam flowing through them, they don't need you making matters worse by sending these huge attachments. Stick to what email was designed for... Sending basic TEXT messages. No fancy formatting, no file attachments, and nothing else that will bloat the size of the message. If you so much as make a single letter bold in a message, it can cause the total size of the message sent expand around 2X, just by way of how email programs work. A nice easy to manage 1-2K message suddenly becomes 2-4K, and if the millions of people sending emails all over the world all do that, you're looking at exponential growth.

You want to do something like this, use a distribution method that is better adapted to this sort of thing, like HTTP. Get a free website from one of the numerous places that offer them, and set up a web page with what you would have normally emailed. People can then go to this website as they please, and aren't stuck downloading huge files through email.

Finally, you need to stop associating file extensions with file types, because the two have no relation whatsoever. Just because some file has a particular extension does NOT automatically mean that's the kind of file it is. I could rename a Word document with the extension .mp3, but it wouldn't make it a music file. A lot of people say something like "AVI codec" when there's no such thing. AVI is a container format, within which is often video encoded with one codec, and audio encoded with another. In all of your specific examples, the file extension is pretty much the same, but it's not always going to be the case.

Anyway, PLEASE, PLEASE, PLEASE stop clogging up email servers everywhere with pointless messages like you are. Make web pages instead, and let people see your work that way. Web servers tend to be run on much "beefier" systems, in terms of hardware, compared to mail servers. They are much better equipped to handle that sort of thing. Any time you've had to wait an especially long time for a message to arrive or you never got a message someone sent you, you can bet there's a good chance it was due to someone sending a bunch of large attachments and clogging up some email relay server. Possibly causing it to crash, making life difficult for the person whose job is to maintain it.