Unfortunately I do not hear anything on the recording — that being said, nose whistling/general respiratory “struggles” are common indicators of an upper respiratory infection, and it is common for veterinarians to start antibiotics for treating it rather than waiting for a culture to come back or something like that. URIs are relatively common in rabbits. Did your rabbit show any signs of improvement on the antibiotic?
Nose whistling can be normal, especially if only happens in times of “relaxation”, like going to sleep or lounging after exercising/being active. Air is being pushed back and forth more forcefully through the nostrils, which can create a whistle sound. If your rabbit is showing signs of breathing difficulties, like open-mouth breathing, excessively rapid breathing for no reason, or there are other signs of a URI, including sneezing or nasal discharge, further veterinary assessment is recommended.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.