I found a really interesting explanation of sound expressed in decibels and distance here
Initially, prosecution said that a witness heard 'shouting' and 'shots' from 600m away. 'Shouting' is roughly equivalent to 80dB. If you are 1 metre away, you'll hear that shout at an equivalent to 80dB...you're right next to them. Every time you double your distance, the 'perceived' dB reduces by 6. Meaning, that by the time you are 4 metres away, you'd 'hear' 68dB, even though the shout is 80dB, because you'd doubled your distance twice.
If the witness was 600m away (1968ft) then the dB she 'hears' the shot at would be around 94dB - so louder than a shout - more like a lorry passing by.
If she were 300m away (984ft) then the dB she 'hears' the shot at would be around 99dB - a motorbike revving.
So, it's very likely she could hear the shots.
However, looking now at 'shouting':
From 600m away (1968ft) shouting (80dB) is 'heard' as 14dB. Now, whispering in a quiet library, 3 feet away is listed as 30dB. Normal breathing is 10dB
From 300m (984 ft) away shouting is 'heard' as 20db - the sound of a mosquito
Source of sound levels
Does it honestly sound likely that she heard an argument, even from 300m away??