It can be a rehearsal, a concert, a regular religious service or even a wedding or baptism.
As @phoog mentions in his answer, if there is nobody at the door to ask your money you can enter.
When there is a service going on, it will be appreciated if you keep out of the area where the people for the service are, in a busy service please stay near the door, if it is a small group in a side chapel or near an altar, you can move around in most of the church.
In the case of a concert, you are asked to stay out of the area unless you sit down and start being one of the group.
Rehearsals are less formal, just do not mingle into the choir or go between the choir and the people which are with them (likely in the seats near.)
Look for notices, they should be obvious but can be to the side or on the other door from the one you used.
And keep your noise down, even more than you would in an empty church.
There are some differences between churches, more between denominations than between different countries. And there are also differences between city and countryside. You can expect a church in a city to be more open to tourists, in the countryside there are still churches which are open every day out of tradition.
Some denominations are more likely to have churches open than others, like Roman Catholic churches have a good tradition of being open for people to come in to pray, while most of the protestant Christian churches are only open for services and to admit tourists. But this is not a given, as each church will decide its own opening times.