I thought mp3 encoding doesn't have a uniform sampling rate, how then
sox --i shows sampling rate field for mp3s? I see 44100 sampling rate for an mp3 file.
Simply, your understanding is incorrect. MP3 does use a uniform sampling rate, it reduces resolution of the sound in areas where the resolution is unneeded so that it fits a simpler pattern that can be more efficiently compressed, but it decodes to a fixed sample rate.
As EMV was kind enough to point out, you may be thinking of the bitrate. The bit rate is the amount of data that it takes to encode those samples. Since resolution is selectively dropped, the amount of data required to store each sample is variable and thus the actual data rate will vary, even though the sampling frequency itself does not.
I suggest the Wikipedia page on MP3 for further reading.