[Fix] Relative Audio Paths
Signed-off-by: stevehuang52 <heh@nvidia.com>

This PR fixes the second problem in this [issue 4455](https://github.com/NVIDIA/NeMo/issues/4455).

Previously, the code will fail when there are "/" in the filepaths of tarred datasets. Now we only add the manifest directory to the audio filepath if the resulting file exists, thus fixing the problem.
