I used mpTrim* 2.13 on some mp3 coded with codec LAME 3.97. If I use foobar2000 (version 0.9.4.4) to play the files, the length shown is the length of the entire file before trimming.
Here there's an example for reference:
http://www.megaupload.com/?d=DYBS85B7

foobar mistakes the length only on files codec with LAME CBR (constant bit rate); with VBR and ABR it'is all ok.
I tried the versions of LAME 3.92, 3.96.1, 3.97 final, 3.98 beta 5, 4.00 alpha 14. (Maybe the version 3.88 isn't affected by the problem, but I'm not sure, and anyway this last seems not a good codec.)

Why this happens?

I wait for a your gentle reply.
Thanks.

I wrote to the author of mpTrim and he given his helpfulness to solve the problem: if he will know where foobar gets the length info, he might be able to update it in mpTrim (his words).


(*) mpTrim is a software that cuts the mp3 files without re-encode

Most problem with mp3 can be fixed by right clicking on the song Utils and choose Fix VBR mp3 header... or Rebuild MP3 Stream.

Effectively the function Fix VBR mp3 header seems fix the problem, and I thank you.
But why this happens, and only in some conditions?
I'd like listen some word by the authors of foobar or by who knows something of more specific.
The authors of foobar pass by here?

foobar2000 reads the exact length from the VBR MP3 header; see also Lame And Xing Header Specs for documentation about this header.