Python convert duration in seconds to hh:mm:ss:ms

[snippsat]

time.gmtime() dos not have milliseconds.
The term is seconds since the epoch: January 1, 1970, 00:00:00 (UTC).

>>> time.gmtime(audio.info.length)
time.struct_time(tm_year=1970, tm_mon=1, tm_mday=1, tm_hour=0, tm_min=3, tm_sec=53, tm_wday=3, tm_yday=1, tm_isdst=0)

>>> time.strftime('%H:%M:%S', time.gmtime(audio.info.length))
'00:03:53'

Don't use len as variable name.
Name it's used bye Python.

>>> len('hello')
5