fix problem with mono and no channels

app.py

@@ -12,6 +12,8 @@ def extend(audio):
     fs, x = audio
     x = x[:int(MAX_LENGTH * fs)]
     x = x.astype(np.float32) / 32767.0
+    if len(x.shape) == 1:
+        x = x[:, np.newaxis]
 
     with torch.no_grad():
         y = np.stack([model(torch.from_numpy(x), fs) for x in x.T]).T