With Audacity for Windows, although I have the lame library installed, Audacity tells me that I need to convert M4A files into WAV in order to import them. Audacity for Mac does not show this message. Why is this so?
You don’t have to “convert” them first.
If you haven’t done so already, download and install [u]FFmpeg[/u]. It adds support for many/most audio formats.
FYI - Audacity works with uncompressed PCM data so when you open a compressed file with Audacity (and with FFmpeg when needed) it gets decompressed. If you re-export with lossy compression (MP3, M4A, etc.) save you are going through another generation of lossy compression. Ideally, you should start with a lossless file and compress ONCE. If that’s not possible, try to minimize the number of times it’s compressed.
LAME is ONLY for creating (or exporting to) MP3. It’s not used for creating M4A files and it’s not used for opening/decompressing MP3s.
I can see the question marks floating over your head. There are two different FFMpegs. 64-bit and 32-bit. I don’t think you can cross them. The fact that just your Windows doesn’t work can mean it’s the wrong one.
Koz