have you tried any web search engine to look for "convert mp3 wav"?
There are so many tools available - even free and open source.
Use any of them with a SysExec call from LabVIEW!
There is no native built-in function to convert any audio format into *.wav format, but, as you might have found out, LabVIEW is able to handle *.wav files. So you need a third party tool to convert the audio files. Follow Gerd's advise...