That resistor limits the amount of current draw
This "mini" (not Pro) web site mentions the delay after starting the next track and the busy state.
// MP3 DFPlayer notes: clones, noise, speakers, wrong file plays and no library – Garry's blog
1kOhms or 2kOhms, same results : clipping noises still there.
I guess it ain't the right module to make seamless audio effects... My best solution so fare is to command to play a new file about 600ms before the current file is playing, that where it seems I got the less clipping sounds/delay on my audio effect transitions.
Since there is no command to know the playing state in this library (well there is one but not reliable and causing 2000ms delay), I use trackers to know when I commanded to play the file and check it against the file length to know where I'm suppose to switch files for a mostly seamless transition between audio effects. Also, since the getTotalTime() only return seconds, I need to create variables for ms tracks lengths in my sketch to be more precise about the switching.
My conclusion is that this module maybe great for playing tunes, but not for sound effects. You need a lot of twisted coding to manage the sound effects correctly.