There are lots of ways to do audio for digital signage. The simplest is to use the speakers in your tv. For those applications where your staff is closer to the tv than your visitors, it is best to put in a separate audio system.
The best option for quiet locations like banks and lobbies is to use pendant speakers to drop down from the ceiling.
An inexpensive amplifier using normal speaker wire like this can drive two speakers with 150 foot or less runs. Some low cost hanging speakers (under $100 each) work well for low volume audio. The PYLE PRJS66W pendant (white) is actually at $29 each – crazy sale on that one. The black one is $62 each. Those have 70 volt transformers – which may require electrical conduit depending on your local building codes.
The 25/70 volt systems use much lighter gauge cable (18+ gauge), so much less costly to install and does not lose much signal over longer lengths. It also adds them all in parallel, so it does not matter (much) how many speakers you put in. The audio quality will be somewhat lower than hi-fidelity speakers, but probably not noticeable for low volume applications like digital signage.
The Speco SP30PTB does 25v or 70v and has a woofer and a tweeter – and a support chain, so probably pretty reasonable sound quality. It looks like it has a volume control on the speaker too which would make audio balancing easier. To be safe, you should select the 25 volt system to prevent the costly requirement to run electrical conduit to the speakers.
This looks like an $82 amplifier with 25 and 70v outputs from BH Photo and Video (a reputable site). Here is a diagram of the 70v or 25v diagram (came from this page – http://www.ticcorp.com/25v_70v_100v_systems.htm):
Most of the pendant speakers already have the line transformer built in, so you don’t have to worry about that.
I feel that multiple pendant speakers above your viewer’s heads is the best – and with the 25v system, you can add more of them and reduce the volume to prevent employee distraction.