Analysis lead to,
a. MP3 frame data may contains sync word patterns (MPEG1, MPEG2 or MPEG2.5 layer 3). How to distiguish betweeen MP3 fomats?

b. Can MPEG1-L3, MPEG2-L3 and MPEG2.5-L3 frames in a single MP3 file. I think it is not allowed because of different sampling rate.

c. Can MP3 file start without sync word?

a.
There's a 2 bit MPEG-ID in the 32 bit mp3 frame header. (first 11 bits of this header is the sync word with all 11 bits set)

b.
You could produce such a file by concatenating separate files. However, it has not been specified what a decoder should do if it encounters such a stream.

c.
Sure. Most ID3v2 tagged files fall in this category


BTW: It has not been specified what a legal MP3 file is. Usually it's a bunch of raw mp3 frames (constant MPEG ID) with optional meta data at the start or end (ID3, Ape-Tag and the like)


SebastianG