Looking for local music ...

I know the feeling

Nubeluz,
You're right, there are plenty of concerts everywhere and all the time but they are hard to find if you only spend a little time down there.
All you can do is indeed watch the billboards along the highways and in town centers, listen to local radiostations, write down the name of the band and of the disco/carwash. Right now is certainly way to early to ask someone, the organizers only start advertising about 10 days beforehand.
The area you mention is a very big one: POP to Santiago. There will definitely be a lot going on when you are there, mostly on weekends, but I think it's better to concentrate on a smaller area when you are only there for one week, and find out what's happening.
Let's see what I remember about merengue/bachata places with live music between POP and STI:
Around POP:Every Sunday afternoon/evening there is live music at a local disco (Terramar?) in Cangrejos on the highway from POP to Sosua. Not the top bands but category of Jos? el Calvo, Pedrito Reynoso, Caballito de Mao.
In Montellano, also between POP and Sosua, disco Brisas de Camu normally has a live concert once a week, I think on Saturdays but not sure. They usually have bigger names.
In POP the Ojilvi Carwash used to have weekly concerts on Mondays, and maybe also Mountain View does something similar.
In Santiago there is a lot going on, especially merengue tipico. When I passed there in July, the city was full of advertising for local bands and also bigger ones. A good internet radiostation from Santiago is www.superregional.com, they also announce who's playing where.
Especially north/west of Santiago you find a lot of merengue tipico 'ranchos':
along Autopista Duarte near Villa Gonzalez are 'Rancho Merengue' and a couple of others, and in tiny Jacagua you have a street full of tipico discos like 'El Tipiquito'. Concerts guaranteed but only on weekends.
You can also check the websites of the artists, some of them list their tourschedule:
www.krisspy.com / www.lostorosband.com / www.rubbyperez.com.do / www.frankreyesonline.com / www.elprodigio.com, etc
Have fun!
Bartolomeo