Is the leading structure in MP3 file a real frame?

MP3 streams don't have a file header. It sounds a bit odd that you have just one frame at the beginning which is longer than the rest, but this is perfectly legal.

There's a quick description of the bits in the header at: http://www.datavoyage.com/mpgscript/mpeghdr.htm

In your case, both types of header share in common:

• MPEG-1
• Layer 3
• Not protected
• 44.1kHz
• No padding
• Not private
• M/S joint stereo
• No copyright
• Original media
• No emphasis

The first frame differs from the rest with:

• 128kbit (resulting in the 417 byte frames minus 4 byte header)

The rest are:

• 32kbit (resulting in 104 byte frames minus 4 byte headers)

There's a formula in that page for calculating frame size based on header: 144*bitrate/samplerate+padding.

I suspect the 128kbit first frame is an artifact (bug) of the encoder used to generate the sample. It's still a constant bitrate file at 32kbit after the first frame. Given that an MP3 decoder can't produce output until it has a few frames, and it won't suddenly hit a bump in bitrate half way through, this is unlikely to upset anything.