My goal is to create original MP3 files with as little quality loss as possible.
I use Audacity on a PC w/ Windows 7 and work with an Apogee mic w/ lightning input into an iPad, recording on Garageband.
Would it be better to get a USB adapter for the mic so I can record directly into Audacity - or upload the MP4 files from Garageband to one of those free conversion websites that turn them into WAV or AIFF files, and email/download them?
You would get better quality recording as uncompressed audio and then converting directly to MP3 format than going through an intermediate conversion of MP4 format.
How good the actual end result of recording directly into Audacity is will largely depend on the microphone/interface that you use.
Does your Apogee mic only have a lightning connector, or can you also use it as a conventional microphone?
It’s actually the 30-pin with a lightning adapter, but I could get a USB cable to plug directly into the PC
That would negate the need for the iPad and as you say would allow for uncompressed recording.
??? What does that mean? 
Sorry. The answer is yes I can use it as a conventional microphone once I get a different cable.
The part you asked about just means the mic came only with an old 30 pin Apple connector and so I needed a lightning adapter to fit the newer iPad.
#unnecessarydetail
What I need to know is what type of audio connector the microphone has, and how the microphone is powered.
the mic is a USB and powered directly from the USB port
OK, thanks.
I’ve read through this several times, and I can’t work out exactly what you are trying to do.
The title says: “Converting Garageband files”, but then you are talking about your USB microphone. I don’t see the connection between the two. Why don’t you just export from Garageband?
As I understand it, maskrcr wants to edit the files in higher quality than MP4.
On OS X, you can use the “Share” menu item in GarageBand to send the recording to iTunes as AIFF which is lossless, then you can import the AIFF into Audacity. I don’t know if GarageBand on iOS has that menu item when you are syncing with iTunes.
Gale