Have you gone back to the fx_afterburner.fx that has the sound or using the gauge based AB_Start.wav.
If your using the fx_afterburner_no_sound.fx The AB_Start.wav plays the AB pop/boom.
This is way harder than it needs to be.
There are only 2 ways to do the afterburner sounds.
1# Use the fx_afterburner.fx in the aircraft.cfg and have the AB pop/boom play any time you hit "L"
2# Use the fx_afterburner_no_sound.fx in the aircraft.cfg and have the dsd_xml_sound3.gau play the AB_Start.wav
For #1 you need use the fx_afterburner.fx in the aircraft.cfg and to delete the AB_Start.wav in the panel.
For #2 do nothing as the F98B_2.0_Preview.zip is set up this way.