I think the problem is that in the audio you can't hear (or I can't hear) the difference between sin mus and sine mus and therefore without knowing in advance what the translation is, unless you can hear properly, you can't differentiate between "its mouse" and "its mice" ???
In reply to NightKnight5 (a direct reply is not possible): Your question contains the answer. From the Danish sentence we know that "mus" is plural because it reads "sine mus". If it there was only one mouse, the sentence would read "sin mus". Accordingly, the English sentence is "its mice" and not "its mouse".
Whether you use mit/min/mine or dit/din/dine depends on what the word ends with, and if it's singular/plural. For example, æblet (the apple) ends in '-et', so you would say mit æble (my apple).
In this case, you know it's not 'sit' because 'the mouse' is 'musen,' which ends in a '-en'; therefore it must be 'sin' or 'sine' depending on context/pronunciation.
(This is known as "noun gender", where nouns whose definite forms end in '-en' are common gender and nouns ending in -'et' are neuter gender).
You're perfectly correct.
This made me aware of a quirk of sorts: while it's "mit æble" and "min mus" ("my apple" and "my mouse"), it's "mine æbler" and "mine mus" in plural. That is, "min" and "mit" both become "mine" in plural. The same applies to "sin" and "sit" which become "sine" in plural. So we have two singular forms, but a single plural form for these words.
It is weird to think the mice are this here cat private property. replace mus with dreng in the sentence to see what I mean. It works with food in the sense of cat food but it doesn't change the fact the mice don't belong to cats. At best they belong to the house, or to their breeder if they are pet mice.
In reaction to Xneb's answer to megmccaliano (which I can't answer to), I was presented with the danish sentence, so Xneb's comment on the english sentence being plural is moot.
As noted by bjarkehs, mus is both plural and singular, so I should have been able to choose between sin, sit and sine, not just sit and sine...
Sorry, when writing the original comment I forgot about the form exercises and assumed it was a translation exercise from English to Danish.
For some reason only one option can be marked as correct. As the original sentence uses "sine", this one cannot be removed so "sin" is not included as this would be marked as incorrect even though it is grammatically correct.