There are many ways to approach this business, just be sure you pick the right model for your temperament. You can get into the recurring revenue model, especially for security and networking. In this case you'll need a ton of installations, each providing a small monthly income. You'll need classic marketing in order to build a large customer base.
On the other side of the coin is the relationship model and referrals. The is where we live. Rather than scatter, the cats will hang out with me. Customers will drop off a key, ask us to install a TV in the bedroom, then leave for vacation.
In general, if you do A/V stuff you'll need to figure out a way to deal with the perception that this equipment will never break, when it does break, it is your fault, and phone support should be free and unlimited. Be prepared for phone calls during your party.
Unless you already have a suitable network, it will take time and some traditional marketing to build a network of referrals. Contractors and designers can be both good and bad referrals. Both can want to be in control and might expect a piece of the deal -- especially designers. We are not against using a contractor's electrician to rough wire. We've found some that do a decent job and this allows us to take on jobs at the edge of our service area without accumulating a bunch of nuisance travel time. Contractors love us because we don't cause any pain on the job. They are not afraid to leave us unsupervised and have us lock up the site as we leave.
And, be prepared to deal with the (true or not): "I can buy it cheaper on the Internet".
