No sound when using sleep fct or watchdog

Using Arduino Audio
1

Heello everybody,
I designed a PCB with an Atmel Samd21 (Pins are similar to MKRZero) which is supposed to play .wav files over the amplifier TAS5805 (connected by i2s). The .wav file is run by DMA (data from SD card).
Everything works fine and I can hear my selected sounds(user Input 'b'). But when I run a watchdog timer (user Input 'c') or sleep function (user Input 'x'), I can't hear my sounds after that, even when I disabled the WDT before again (user Input 'v').
I guess there are some troubles with the RT clock (code is still running properly, even after sleeping). Did someone have similar problems and knows how to solve it? I am grateful for every advice/approach.

Also I appended a test code. Maybe that's helpful to understand what I am talking about.

Thank you!

#include <SPI.h>
#include <SD.h>
#include <ArduinoSound.h>
#include <RTClib.h>
#include <TAS5805.h>
#include <Adafruit_SleepyDog.h>
#include <ArduinoLowPower.h>

//Amplifier/ Audio
class TAS5805 TAS5805_AMP;
SDWaveFile waveFile;

//RTC
RTC_PCF8523 rtc;

//SD Card 
volatile int SDCARD_CS_PIN = 28;

bool wdt_on = false;

void setup() {
  Serial.begin(9600);
  while(!Serial){};
  
  pinMode(8, OUTPUT); //PIN_TAS5805_SHDN =8
  digitalWrite(8, HIGH);
  
  
  //initialize SD card
  SDcardInit();

  TAS5805_AMP.powerDown();
}

void loop(){

  
  if(Serial.available()){
    char c = Serial.read();
    
    if(c=='x'){
      LowPower.sleep(5000); //sleep for 5 seconds
    }
    else if(c=='c'){
      Serial.println("Enable WDT");
      Watchdog.enable(1000000); //set Watchdog (1000 seconds timer)
      wdt_on=true;
    }
    else if(c=='v'){
      Serial.println("Disable WDT");
      Watchdog.disable();
      wdt_on=false;
    }
    else if(c=='b'){
      playFile("test.wav",0); //play single
    }
    else if(c=='n'){
      playFile("test.wav",1); //loop
    }
    else if(c=='m'){
      Serial.println("stop playing");
      AudioOutI2S.stop();
    }
  }
  if(wdt_on)
    Watchdog.reset();
}

void SDcardInit(){
  if (!SD.begin(SDCARD_CS_PIN)) {
    Serial.println("SD Card Error");
    while (1);
  }
  Serial.println("SD Card detected");
  delay(3000);
}

void playFile(String s, bool loopMode){
  
  waveFile = SDWaveFile(s);

  // check if the WaveFile is valid
  if (!waveFile) {
    Serial.println("WAV invalid");
    delay(3000);
  }
  else{

    Serial.println("found valid file");
    
    TAS5805_AMP.begin(0x2D); //0x2D -> slave address

    TAS5805_AMP.setVol(5);//50% volume from amplifier
    
    // full volume from MCU
    AudioOutI2S.volume(100);

    // check if the I2S output can play the wave file
    if (!AudioOutI2S.canPlay(waveFile)) {
    Serial.println("unable to play!");
    delay(3000);
    }
    else{
      Serial.println("play file");
      if(loopMode==0){
        AudioOutI2S.play(waveFile);       
      }
      else{
        AudioOutI2S.loop(waveFile);
      }
    }
  }
}