Remove reference to sox in CONTRIBUTING.md

[samanklesaria]

I missed one last reference to sox: it's in the contributing.md file. This PR removes it.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/audio/4101

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❌ 11 New Failures

As of commit 7f4912e with merge base fa51429 ():

NEW FAILURES - The following jobs have failed:

• .github/workflows/integration-test.yml (gh)
• Build Aarch64 Linux Wheels / pytorch/audio / build-wheel-py3_10-cuda-aarch6413_0-aarch64 (gh)
• Build Aarch64 Linux Wheels / pytorch/audio / upload / upload-wheel-py3_10-cuda-aarch6413_0-aarch64 (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_audio__3.10_cu130_aarch64
• Build Windows Wheels / pytorch/audio / build-wheel-py3_10-cuda12_8 (gh)
  Process completed with exit code 1.
• Build Windows Wheels / pytorch/audio / build-wheel-py3_10-cuda13_0 (gh)
• Build Windows Wheels / pytorch/audio / upload / upload-wheel-py3_10-cuda12_8 (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_audio__3.10_cu128_x64
• Build Windows Wheels / pytorch/audio / upload / upload-wheel-py3_10-cuda13_0 (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_audio__3.10_cu130_x64
• Unit-tests on Linux CPU / tests (3.11) / linux-job (gh)
  RuntimeError: Command docker exec -t 68a0724411d6114367d64b120fb685260b2d77a6c3ad5bbe0d9b37da3f4ff523 /exec failed with exit code 1
• Unit-tests on Linux GPU / tests (3.10, 12.6) / linux-job (gh)
  RuntimeError: Command docker exec -t d2d79e63b0f240644d6516343149093a34d1541c59dd2bda1ddcd55d1bf85fd0 /exec failed with exit code 1
• Unittests on Windows CPU / unittests-windows-cpu / windows-job (gh)
  Process completed with exit code 1.
• Unittests on Windows GPU / unittests-windows-gpu / windows-job (gh)
  Process completed with exit code 1.

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[pearu]

LGTM!