treat 385 mp3 files

I should want to increase the volume of 385 mp 3 files( 3,52 go).
Is it possible to treat them simultaneously with Audacity, or do i have to separate the files to be able to treat it??.
Thanks for your helping.

Firstly, MP3 is a “lossy” format.
In order to make the files small, MP3 encoding throws away some of the audio data and then packs the remaining data together tightly. Some audio quality is always lost when encoding audio into MP3 format.

For Audacity to be able to do all of the cool things that it can do, it requires that the audio is uncompressed - this means that when you import an MP3 file it is decoded into uncompressed data. In turn this means that if you want the final edited file(s) in MP3 format, then it must be re-encoded in MP3 format and this will lose some sound quality (because MP3 encoding always loses some sound quality).

On to your question:

Audacity is able to apply one or more effects/processes as a “Chain” of effects to multiple files. All of the files must be in the same directory. So you could create a “Chain” that Amplifies (or Normalizes) and then exports the amplified version.

However, given the preamble about losing sound quality, it may be better to use a program that can change the volume without needing to decode the MP3.
I don’t use Mac OS X myself, but some programs that came up with a quick Google search:
Sound Check

Thanks for your answer Steeve.
As i don’t want to harm my mp3 files cause i play it on stage, i 'm gonna try one of the softs that you enumerate, in order to not convert and reconvert my files( excuse me please for my approximative english language, cause i’m french).
I also get a PC so i can treat my files with softs that are usefull on PC.
If you know some of them, can you tell me their names please.

I’m not really in a position to recommend specific software, though I would recommend that you always look for open source options.