Fix label string assignment in audio pipeline

[qanastek]

@sergioburdisso Urgent PR to adapt to the new format of the foreground and background datasets.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.79%. Comparing base (dc54487) to head (c1c5511).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #121   +/-   ##
=======================================
  Coverage   53.79%   53.79%           
=======================================
  Files          41       41           
  Lines        6746     6746           
=======================================
  Hits         3629     3629           
  Misses       3117     3117           

Files with missing lines	Coverage Δ
src/sdialog/audio/pipeline.py	53.40% <100.00%> (ø)

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