it sounds as though you have a good relationship with these neighbours and that both you, and they, have been considerate towards each other in the past. Given that history, I would go round one day after a party and explain that while you appreciate that they want to enjoy their garden, there is an awful lot of noise, which is very audible and disturbing, and ask them whether they could keep the noise down, particularly later in the evening and when they have parties during the week.
If no-one has mentioned anything to them, they may not be aware that the noise they make is bothering anyone. different people have very different ideas about what is a normal level of noise.
I think that if you already had problems with them being ant-social or aggressive, then calling the police rather than approaching them, direct might be appropriate, but as it is, the hostory you have with them is one of mutual consideration and courtesy.