by the way you can merge the files directly without taking care
about header, cause most of mp3 player fix the header errors.
but it is much more better to deal with header and format as Richard MacCutchan said.
how ever here is a code which merger them directly.
byte[] a = File.ReadAllBytes(txt_a_path.Text);
byte[] b = File.ReadAllBytes(txt_b_path.Text);
byte[] c = new byte[a.Length + b.Length];
a.CopyTo(c, 0);
b.CopyTo(c, a.Length);
File.WriteAllBytes(txt_c_path.Text, c);
and this is the sln link:
http://imljh.ucoz.com/code/Merge_mp3_no_header_care.zip