```python
import subprocess
import os

def list_audio_files(directory):
    """
    Lists all audio files in a directory.
    :param directory: the directory where files are stored.
    :returns: the list of files paths
    """
    audio_files = []
    for root, dirs, files in os.walk(directory):  # os.walk will iterate through all the folders recursively
        for file in files:
            full_path = os.path.join(root, file)
            result = subprocess.run(['file', '--mime-type', '-b', full_path], stdout=subprocess.PIPE)  # this calls a system command
                                                                                                       # returning the MIME type of the file
            mime_type = result.stdout.decode().strip()  # this cleans the MIME type for comparison
            if mime_type in ['audio/mpeg', 'audio/x-wav', 'audio/mp4', 'audio/x-m4a']:  # here are some frequent MIME type for audio files
                audio_files.append(full_path)
    return audio_files

directory = '/path/to/directory'
print(list_audio_files(directory))
```
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