bugfix: use parameter mono only if passed explicitly

Caused the tests/test_separation to fail.

src/freqsplit/input/file_reader.py

@@ -1,7 +1,7 @@
 import os
 import librosa
 
-def read_audio(file_path, sr=None, mono=False):
+def read_audio(file_path, sr=None, mono=None):
     """
     Reads an audio file and returns the audio time series and sampling rate.
 
@@ -17,7 +17,11 @@ def read_audio(file_path, sr=None, mono=False):
     if not os.path.exists(file_path):
         raise FileNotFoundError(f"File not found: {file_path}")
     try:
-        audio, sr = librosa.load(file_path, sr=sr, mono=mono) # Load with original sampling rate.
+        librosa_kwargs = {"sr": sr}
+        if mono is not None:  # Only add 'mono' if explicitly provided
+            librosa_kwargs["mono"] = mono
+
+        audio, sr = librosa.load(file_path, **librosa_kwargs)
         return audio, sr
     except Exception as e:
         raise RuntimeError(f"Error reading the audio file: {e}")