They are incompatible since the frames are not decoded, so they can't be preprocessed.
Originally committed as revision 14178 to svn://svn.ffmpeg.org/ffmpeg/trunk
codec->time_base = ist->st->time_base;
switch(codec->codec_type) {
case CODEC_TYPE_AUDIO:
+ if(audio_volume != 256) {
+ fprintf(stderr,"-acodec copy and -vol are incompatible (frames are not decoded)\n");
+ av_exit(1);
+ }
codec->sample_rate = icodec->sample_rate;
codec->channels = icodec->channels;
codec->frame_size = icodec->frame_size;