diff --git a/twang/source_separation/nussl.py b/twang/source_separation/nussl.py
index f99590b..2874188 100644
--- a/twang/source_separation/nussl.py
+++ b/twang/source_separation/nussl.py
@@ -1,7 +1,7 @@
 import nussl
 
 from twang.source_separation.base import SourceSeparation, SourceSeparationDict
-from twang.track import BaseTrack
+from twang.track import BaseTrack, LibrosaTrack
 
 
 def _track_to_nussl_audio_signal(track: BaseTrack) -> nussl.AudioSignal:
@@ -9,6 +9,10 @@ def _track_to_nussl_audio_signal(track: BaseTrack) -> nussl.AudioSignal:
     return nussl.AudioSignal(audio_data_array=librosa_track.y, sample_rate=librosa_track.sr)
 
 
+def _audio_signal_to_librosa_track(audio_signal: nussl.AudioSignal) -> LibrosaTrack:
+    return LibrosaTrack(audio_signal.audio_data, sr=audio_signal.sample_rate)
+
+
 class Repet(SourceSeparation):
     """https://nussl.github.io/docs/separation.html#foreground-background-via-repet"""
 
@@ -18,7 +22,10 @@ def run(self, track: BaseTrack) -> SourceSeparationDict:
         audio_signal = _track_to_nussl_audio_signal(track)
         repet = self.repet_cls(audio_signal)
         estimates = repet()
-        return {"background": estimates[0], "foreground": estimates[1]}
+        return {
+            "background": _audio_signal_to_librosa_track(estimates[0]),
+            "foreground": _audio_signal_to_librosa_track(estimates[1]),
+        }
 
 
 class RepetSim(Repet):
diff --git a/twang/track/base.py b/twang/track/base.py
index f680b4e..888ca8c 100644
--- a/twang/track/base.py
+++ b/twang/track/base.py
@@ -121,6 +121,7 @@ def _repr_html_(self) -> str:
 
     def __getitem__(self, ms_or_slice: Union[int, slice]):
         """TODO: test & document"""
+        self.file_path = None  # because the object no longer corresponds to the file on disk
         self.y = self.y[ms_or_slice]
         return self