Audacity Portable can open AU, AIFF, MP3, OGG and WAV files and can save to WAV, OGG or MP3 (with the free lame plugin).
it's a full blown audio editor as well, but you can easily use it just for...

Solution 1: No Internet Access In Windows 8.1or Windows 10

Step 1:

Go to Device Manager → Network adapters.
Step 2:

Expand it (Network adapters).