here's the code, with 2 mono wav files merged to stereo, at 22050hz samplerate
where sounds play fine :

#include <SPI.h>
#include <SD.h>
#include "AudioTools.h"
#include "AudioTools/AudioLibs/AudioBoardStream.h"
#include "AudioTools/Disk/FileLoop.h"

const int chipSelect = PIN_AUDIO_KIT_SD_CARD_CS;

AudioInfo info(22050,2, 16);
InputMerge<int16_t> merge;
AudioBoardStream i2s(AudioKitEs8388V1); // final output of decoded stream
FileLoop loopingFile;
FileLoop loopingFile2;
EncodedAudioStream decoder(&loopingFile, new WAVDecoder());
EncodedAudioStream decoder2(&loopingFile2, new WAVDecoder());

StreamCopy copier(i2s, merge, 256); // 256-byte buffer

void setup() {
Serial.begin(115200);
AudioLogger::instance().begin(Serial, AudioLogger::Warning);
auto config = i2s.defaultConfig(TX_MODE);
config.copyFrom(info);
config.sd_active = true;
config.buffer_size = 256; // 256-byte I2S buffer
config.buffer_count = 2; // Double buffering
i2s.begin(config);

SPI.begin(PIN_AUDIO_KIT_SD_CARD_CLK, PIN_AUDIO_KIT_SD_CARD_MISO,
PIN_AUDIO_KIT_SD_CARD_MOSI, PIN_AUDIO_KIT_SD_CARD_CS);
SD.begin(chipSelect, SPI);
loopingFile.setFile(SD.open("/pinaVoix/voxNoise.wav"));
loopingFile2.setFile(SD.open("/birds/weirdBirds.wav"));
loopingFile.begin();
loopingFile2.begin();
merge.add(loopingFile, 1);
merge.add(loopingFile2, 1);
merge.begin(info);
decoder.begin();
decoder2.begin();

}
void loop() {
copier.copy();

}

switching to sdmmc solved the issue, here is a sample code:

#include "FS.h"
#include <SD_MMC.h>
#include "AudioTools.h"
#include "AudioTools/AudioLibs/AudioBoardStream.h"
#include "AudioTools/Disk/FileLoop.h"

AudioInfo info(44100, 2, 16);
InputMerge<int16_t> merge;
AudioBoardStream i2s(AudioKitEs8388V1); // final output of decoded stream
FileLoop loopingFile;
FileLoop loopingFile2;
EncodedAudioStream decoder(&loopingFile, new WAVDecoder());
EncodedAudioStream decoder2(&loopingFile2, new WAVDecoder());

StreamCopy copier(i2s, merge, 512); // Larger buffer for stereo data

void setup() {
auto config = i2s.defaultConfig(TX_MODE);
config.copyFrom(info);
config.sd_active = true;
config.buffer_size = 256;
config.buffer_count = 2;
i2s.begin(config);
i2s.setVolume(0.7);

if (!SD_MMC.begin("/sdcard", true, true, SDMMC_FREQ_52M)) {
return;
}

File file1 = SD_MMC.open("/pinaVoix/voxNoise.wav");
if (!file1) {
return;
}

File file2 = SD_MMC.open("/birds/weirdBirds.wav");
if (!file2) {
file1.close();
return;
}

loopingFile.setFile(file1);
loopingFile2.setFile(file2);
loopingFile.begin();
loopingFile2.begin();

decoder.begin();
decoder2.begin();

merge.add(decoder, 1);
merge.add(decoder2, 1);
merge.begin(info);

uint8_t testBuffer[64];
decoder.readBytes(testBuffer, 64);
decoder2.readBytes(testBuffer, 64);
}

void loop() {
copier.copy();
}

Thanks a lot