Hmm.. that term 'Upmixing' makes me think they might mean the problem may be in converting your 10 second audio up to the same format as your movie's audio. This means if your clip is mono and some lower bit rate like 44.1kb/S, Premier may not be able to upmix to stereo and 48kb/S. Or it may be a stereo to 5.1 channel surround sound problem.
If this is the problem, you'll need to convert your 10 second clip externally.
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