The reason there are no responses yet is you have to be more specific. Stereo? Home theater? Room size? Electronics? Budget? If you don't give us more information, people will just suggest what they own, which may not serve your application best.
There are many bright folks here who can give you great advice if they know what you need.
Seriously, in-walls run a gamut of prices. For your 5.1 system, are you thinking $1000, $2000, $5000, $10,000, more ? It really does make a difference for us to recommend something in your $ range. I'd hate to send you out listening to speakers that are way over your budget. That would be a waste of time for you.
Generally, speaking in the mid-range ($200-$500 per speaker) there are many good manufacturers to choose from: Polk, B&W, Klipsch, Niles, Boston Acoustics, etc. I'm personally leaning towards buying B&W in-walls, but that's because of all the in-walls I've listend to so far, I like their sound the best. OK, that's not entirely true, I really liked the James Loudspeaker system I heard, but it was outside of my budget. You might prefer the sound of a different manufacturer.
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