If the mic input to the SIM800L is a true differential type, equal resistors in both signal lines may work better (eg. 10K in each), plus the 1K across the mic input.
And, add the 47 or 100 Ohm directly across the DFPlayer output?
Nigel - the SIM800L datasheet gives signal-to-noise at a signal level of 0dBM0, so around 600mV should be acceptable, presumably via some built in AGC?
It certainly should not need to be at low millivolt levels.
10:1 should be more than adequate reduction.