How to increase volume in ffmpeg?

Question

fmpeg

I have this code:

shell_exec("ffmpeg -i $first_media_path -i $second_media_path -filter_complex \"[0:a][1:a]amerge = inputs = 2\"" .
           " -c:v copy -acodec aac -ac 2 -strict experimental -ab 160k $dir$name");

How can I turn up the volume?
Tried -af but it's not compatible with -filter_complex