[BScout]

Originally Posted by vap80

I suppose a session can handle multiple bit depths, but I cannot find anything on that in the manual or internet (maybe I do not search correctly ). So I just want to ensure that when changing the session's bit depth while working on them it keeps the old clips in the initial bit depth and imports the new files in the changed bit depth.

Yes, your understanding is correct. Old files stay the same and new files are at the new bit depth. There's zero issue mixing bit depths in a session.

Originally Posted by vap80

I always want to check metering to the new exports with izotope insight 2. I do that mostly to check the mp3 version as it is the most common type of file in digital music world, so I want to be as good as possible. Also I'd like to ask what kind of conversion does the Pro Tools to an imported mp3 file. Does it affect the sound?

When you import an mp3 into Pro Tools that is the same sample rate as your session, if you use "Add" with no SRC, the file is unchanged. Pro Tools can handle mp3s as mp3s directly. If you select copy with SRC, then you are creating a new file that will be at the current bit depth setting and sample rate (and be the format of your session and this "copy" file will not be an mp3: BWF/WAV or AIFF).

The conversion from mp3 (lossy) to WAV/BWF/AIFF (all linear PCM formats and lossless) is direct. Nothing is added or subtracted from the sound. It is what all mp3 playback devices do for actual playback (they have to decode to linear PCM.)