Most stores and malls will use an inline array running at 50/70/100 Volts with a VERY bandpassed signal (probably 300Hz-8kHz, not that much more than the audio bandwidth of a telephone call). This allows the signal to be more easily amplified but sacrifices a good deal of fidelity - the speakers will be designed to reproduce the same frequency range for efficiency, but most importantly the intelligibility will be preserved as speech falls mostly into the 500Hz to 6kHz band. A good PA system will also contain signal processing for incoming audio, compressing, EQing and bandpassing the signal to make the most of the system's limited bandwidth.
PA speakers found in shops are usually optimised for directionality and intelligibility over faithful sound reproduction; there's a reason they sound a bit tinny because, at their simplest, they're optimised for reproducing speech in a way that cuts through the general white noise of a store and allows humans to hear it. Off-the-shelf speaker systems or general PA systems won't cut it on their own.
Whilst I haven't had the opportunity to shepherd a large PA system build and installation through to reality, I did spend four years at University getting a degree in sound engineerng and production (and some of it stuck ;-) along with a big dollop of practical experience. I've rigged small and medium PAs in odd-shaped venues so I do have some tips to dispense - I welcome comments from more experienced people or people who do this as a day job!
I'd consider four things as most technically important when speccing a public address system:
- Drivers' ability to accurately reproduce input signal
- Desired directionality of speakers
- Flexibility of installation
- Discretion of system when installed
And in terms of the build itself:
- Physical size of venue and capacity
- Dimensions of room and desired level(s) of amplification throughout
- Intended use of system (you say vocal reinforcement)
- The depth of your pockets
You need to carefully plan out the system if you're covering a warehouse space, compensating for the room acoustic and delays experienced by listeners relative to the speaker's position is critical. Largely empty spaces will boom and reverberate pretty much as a given (with a really long RT60), so don't forget to get some human bodies in the space if you're auditioning speaker placement and listening positions.
If you're after flexibility of installation above all else, you might want to consider the compromise of a more discreet but slightly inferior sound quality 50/70/100 Volt array (similar to what you see in some towns or at railway stations etc); the alternative is a fully-fledged line array akin to what you'd see at music concerts, if your space is huge and will be fully populated during performances.
All options have their pros and cons - I would opt for a line array as (provided you spec it correctly) you can introduce microsecond delays to specific zones to to provide uniform, synchronised amplification of the speaker's voice throughout the venue accounting for propagation delay.
You need to focus clearly on blocking out the PA - separating speakers into zones, speccing the correct amplification and dealing with placement. There are PA systems you can buy from established manufacturers that deal with specific issues of directionality, awkward mounting positions etc (things like slim profile speaker units, or downward-firing drivers shrouded in clear parabolic reflectors to avoid sound spillage outside of a particular floor area).
Your budget will be the first question asked of you if you approach a PA company, I suggest you work with a professional installer if the task isn't a simple plug-and-play 'Mackie on stands' job. (As an aside, Bose manufacture their "L1" system, which is essentially an incredibly thin 'soundstick' paired with a sub for bass reinforcement - you might find that suitable for audience-facing sound reinforcement).
If you want to do it on the cheap - something like Mackie SRM450s - above head height! - will work; getting them above head height on stands and angling them to fire down over the audience is a useful trick as that'll avoid the front three rows absorbing all of the high frequency sound (resulting in a muffled mush for everyone else). Human bodies are a remarkably effective absorber of sound waves. Also, be careful with your placement - make sure you avoid lots of scatter and reflection from parallel surfaces.
Don't forget to have fun ;-)