Catalyst 3750 QOS Configuration

The dscp and cos markings are just that... they're only markings.  You need to specify how you're going to prioritize.

you would apply a service-policy to the port where you want your prioritizing to take place.  for example...

-First, you create class maps.  These are used to identify different types of traffic.
-Next, you create policy maps.  These set a policy to the classes you identified with class-map.
-Finally, apply a service-policy to port(s).  This will share the name of a policy-map you created.

there are many different ways of identifying traffic for a class-map.  Using the '?' will be very helpful.  As, different devices with different IOSs will have different options.  You can specify address ranges, using access-lists and wildcard masks, you can specify to match specific protocols, I think you can even specify vlans... etc...

There are also different ways to prioritize with your policy-map.  typically you would either just specify that a specific class gets a set amount of bandwidth, or that it gets a percentage.

I'll give a simple example of prioritizing 1.024Mbps, so the math is easy.  The following policy will pick up an IP range that is VOIP '10.0.0.0 255.255.255.0' and set it to ef (express forwarding).  This is the priority queue, and you can only have one priority queue.  Then It will set a specified device that is a video suite to the next highest queue (ip dscp 41).  The 41 means it has a cos (class of service) of 4, and a drop precedence of 1... higher drop precedences mean these packets are more likely to be dropped if you turn on something for congestion, such as random-detect.  Per your comment above, a cos of 4 is not for the priority queue, that will be for a cos of 5, which is what VOIP phones are typically given, or allowed to give themselves.  Finally this example will specify a class OTHER, which will be for everything else, and which I will turn on random-detect to drop packets during congestion from this group.  Notice I do not allocate 100% of the bandwidth, this is to ensure I don't guarantee all of my bandwidth to something, and leave nothing for control traffic.

ip access-list extended VOIPmatch
   permit ip 10.0.0.0 0.255.255.255 any
ip access-list extended VTCmatch
   permit ip host 192.168.0.1 any

class-map match-any VOIP
   match access-group name VOIPmatch
   match ip dscp ef
   match ip rtp 16384 16383
class-map match-any VTC
   match access-group name VTCmatch
class-map match-any OTHER
   match any

policy-map MyMap
   class VOIP
      priority 512
      set ip dscp ef
   class VTC
      set ip dscp 41
      bandwidth remaining percent 50
   class OTHER
      set ip dscp default
      bandwidth remaining percent 50
      random-detect dscp-based

interface Serial1/0
   blah
   blah
   blah
   service-policy output MyMap

A couple of those items weren't necessary, just to show you the possibilities, such as matching ip protocol rtp (Real Time Protocol), which is a version of UDP cisco uses for voice and video traffic, I got away with it there, because my video suite is over an encrypted channel, so it won't be recognized as rtp.  You can also match by mac addresses if you want to be anal, but you can only do so on ethernet ports since they're ethernet addresses.  

The maps act just as access-lists or route-maps do, they're read top to bottom.  The match-any means a specific map will pick of traffic that meets any of the match statements, as opposed to match-all.  The access lists I used, are again... similar to those used with route-maps, they don't block traffic, they are just used to identify traffic.

hope that helps.