I mean with my question that I have some mp3 files encode at 48KHz at any bitrate stereo and Vegas 4 say me "Audio: Stream attributes could not be determined; MPEG Layer-3" In the explorer Tab.
If I change the sample rate in another software to 44,1KHz I can add it in my vegas project.
You are confusing MP3 encoding rate with PCM sampling rate. They are completely different things.
You should be able to add mixed sample rate wav files to a Vegas project, and you should be able to add MP3 files. Maybe your MP# file is invalid in some way.
Actually MP3 files do have a sampling rate just like any other audio file. If the bitrate is high enough, MP3 files will be saved with the project bitrate (48KHz or 44.1KHz, for example). If the bit rate is low enough the sampling rate will drop to 1/2 or 1/4 (24KHz or 11.025KHz, for example) so that there is less data to encode.
Nothing to do with it. The media attributes don't have anything to do with the project setting and whether they will play or not. All audio is resampled on the fly to the project sample rate.