I just read a warning against using mp3 in a project. I thought Audacity admitted mp3. I have recent project that contains both wav & mp3. Are there going to be problems from mixing these two formats? I need the mp3 and can replace the wav with mp3 versions. What’s the full story. Can I convert the mp3 to wav?
There is no problem mixing & matching.
As you probably know, MP3 is lossy compression. It can be very good and you don’t always notice any quality loss and if you are stuck with it, you’re stuck with it. 
When you open the MP3 it gets decompressed. If you re-export as MP3 you are going through another generation of lossy compression and the damage does accumulate. So, try to minimize the generations of lossy compression.
If you export to WAV there is no further damage. But if you need MP3 or another lossy format there’s no way around 2 generations of compression.
Can I convert the mp3 to wav?
Yes, but that doesn’t improve the quality so it doesn’t usually help anything (and it won’t hurt). Information is thrown-away during compression (not during decompression).
MP3 is something of a time bomb. Each time you make a new one out of an old one, it gets a little worse. Whoever gets stuck at the end of the chain has to listen to a honky, bubbly show.
We had a poster who was doing a broadcast radio show by downloading MP3 music at home and then mix-announcing commentary and analysis. He would make a new MP3 of the whole show and ship it off to the radio station. The radio station would broadcast it and everybody was happy…until they tried to make their MP3 podcast. The music segments turned into distorted garbage.
We worked with them to eliminate as many MP3 steps as possible. I think he ended up burning a standard Music CD of the show (perfect quality, uncompressed WAV format) and hand-carry that to the station.
MP3 is an end format. Make a show, convert it to MP3, post it, and full stop. There is no pull it down and edit it again. We warn the audiobook people to keep WAV or lossless copies of their Edit Master Reading, because you can’t submit the MP3 posting more than once.
Koz
If you make a WAV copy of an MP3, you have just made a perfect, clear, uncompressed copy of the MP3 plus the MP3 distortion.
Koz
Sounds like mixing wav with mp3 is not an option. Because if people make copies of copies of copies, the mp3 part of the recording is going to disintegrate. 
If I make a wav copy of the mp3 portion of my project, will that disintegrate also with repeated recopying?
Sounds like mixing wav with mp3 is not an option. Because if people make copies of copies of copies, the mp3 part of the recording is going to disintegrate.
Not exactly. It’s OK to mix and match. Once you open it in Audacity it’s uncompressed so it’s no longer an MP3.
Audacity is working with a format that’s neither WAV or MP3, but it’s the same underlying PCM format as 32-bit floating-point WAV.
And if you have an MP3, it’s still digital and you can make copies of copies of copies with no degradation. But if you decompress it (by opening in Audacity, etc.) and then re-compress it (by exporting as MP3, etc.) the damage accumulates with every generation of compression.
BTW - AAC (M4A, MP4 audio) is much-much more immune to that kind if accumulated damage.
If I make a wav copy of the mp3 portion of my project, will that disintegrate also with repeated recopying?
No. The damage only happens when you when you compress (or re-compress) to MP3.
If you want to save/export any intermediate work, export as WAV. And, if you want the “final"production” as an MP3, you might want to save/archive a WAV or FLAC copy.
My standard MO is to record in wav. I sampled some items from some videos I found online, after converting them to mp3. Now I will export from Audacity into wav. This should be OK for people to pass along through copies. But unlike my other wav projects, this project will suffer from being compressed and decompressed?