Questions on Need of filtering before decimation?
If you think for a moment why you filter, it is to remove frequencies that would fold to the remaining bandwidth when decimating.

So if these frequencies that would fold do not exist in the audio then it will have same bandwidth whether you filter it or not. So in the first case, there is really no need to apply LPF first (unless there is noise that would fold and cause problems).

In the second case, it needs to be filtered, and you calculated the cut-off correctly. However, in practice it won't be a brick-wall filter, so depending on needed filter performance and allowable transition bandwith, you might want to select for example cut-off of 4.5 kHz and allow the filter to have about 1kHz transition band to have some reasonable attenuation level at 5.515 kHz.