Fix issue #140: Suppress pydub warnings when ffmpeg is not available

[neubig]

This PR fixes issue #140 by suppressing pydub warnings when ffmpeg is not available.

Changes:

1. Added conditional import for pydub that checks if ffmpeg or avconv is available
2. Modified Mp3Converter to check for pydub availability before attempting to use it
3. Added unit tests to verify the fix

Testing:

• Added unit tests that verify the Mp3Converter handles the case when pydub is not available or ffmpeg is not installed
• All unit tests pass

Fixes #140

@neubig can click here to continue refining the PR

[openhands-ai]

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@OpenHands please fix the failing actions on PR #141

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[neubig]

LGTM need a second review

[ryanhoangt]

LGTM, I test locally and this seems to work!