[Fix] Fixed sampler override and audio_key in prepare_audio_data
I tested the changes on my local branch, didn't face any problems
# What does this PR do ?

This PR is fixing two issues

1) Currently, `process_audio.py` tries to override `sampler` even if `cfg.sampler` is an empty dict (so nothing to override)
2) Currently, `transcribe_speech.py` is crashing if using `audio_key != "audio_filepath"`

**Collection**: audio, ASR

# Changelog 
1) Skip sampler override if `cfg.sampler` is an empty dict
2) In `transcribe_utils`, assign the resolved audio path to `item["audio_filepath"]`

# Usage

N/A

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# Additional Information
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