A2DP-Source can be operated with a sample frequency of either 44100 Hz (standard) or 48000 Hz.
The frequency can be set via menuconfig in the Bluetooth A2DP_Source Configuration submenu.
The solution is based on espressif/esp-idf#1720 (comment) 

Changes were made to the following BT component source files (marked with '// ANP')
    \components\bt\host\bluedroid\btc\profile\std\a2dp\bta_av_co.c
    \components\bt\host\bluedroid\btc\profile\std\a2dp\btc_a2dp_source.c
    \components\bt\host\bluedroid\common\include\common\bluedroid_user_config.h
    \components\bt\host\bluedroid\common\include\common\bt_target.h