The plan is to build a spooky radio. Connected to the arduino should be a pir sensor, two leds and a mp3 sd card shield. When the radio is in normal mode the two leds are on and it should play one track after another. When the pir sensor is triggered it should play one specific mp3 file which is not played normaly and the leds should start to flicker. After someplaying this track/effect it should go back to normal mode.
Every help is welcome.
This is quite easy. Use the DFPlayer module which you can get for less than 1.50€ in China an Arduino (I recommend the arduino nano, since it fits perfectly in size and is easy to connect and use), you pir and leds and a speaker. Use the DFRobot DFPlayer library at https://github.com/DFRobot/DFRobotDFPlayerMini. There you find also the datasheet and links to good documentation for all functions.
The DFPlayer module has a built-in feature for your spooky effect. Use the advertise() command from the above mentioned library to get exactly what you want.
Thank you for this information. I am still waiting for my DFPlayer (Aliexpress) to tinker with it. Hope it is as easy as you think. Don't underestimate my level of dumbness. :D