The sound level of the Baltic at 2m height waves (calm conditions*) is 78dB. The level at which your hearing will be damaged within a minute is 115dB. The threshold safe level - for example, in the workplace or DIY work - is 85dB over 24 hours.
Thus, you can see immediately that waves of a height between 6m - 8m as happened on the accident night would give a decibel level far higher than at calm levels. Survivors said they could not hear what other people were shouting on deck. Bear in mind, decibels is not a linear measure but an exponential one. In other words, for every 3 decibel increase, the intensity of sound doubles, as it is a measure of magnitude (rather like a telescope lens). Thus, water pouring into the car deck at 200 tonnes/per second all on one go into clanking metal would be deafening. The Herald of Free Enterprise capsized virtually straight away so if it was the case then it should have turned turtle within about five minutes.
*The waters of the Baltic are relatively shallow, between 300m to 30 metres deep, with the unlevel seabed giving rise to unexpected currents. Seas have higher waves than oceans in a storm for that reason (Pacific = 'calm').
Higher waves = louder noise.