Oh nice, something I know a bit about. Let me ramble on about this great length.
So, the thing to remember about sound is that it only really interacts with things that are about the same size as its wavelength. As you probably know, wavelength is inversely proportional to frequency, so high frequency sounds interact with little things, and low frequency sounds interact with big things.
For example, if you have a small hole in a wall, low frequency sound will ignore the hole, and reflect off the wall, and high frequency sound will go right through. Alternatively, if you have a small bit of wall standing by itself, the high frequency sound will reflect off it, but the low frequency sound will just spill round the edges, no problem, and carry on past.
What does this have to do with anything?
Well, if you're using sound for echolocation, your resolution is defined by the frequency of the sound you're using. If you tried to use low frequency sound for echolocation, the sound energy wouldn't reflect off anything smaller than the ground or a big wall or something.
Hence, creatures that use echolocation use ultrasonic frequencies.
But, high frequency noise is terrible for communication, because you basically need line-of-sight (also it gets absorbed by the air) so your range is low as hell.
So, you'd need two distinct systems, really.
The same rules apply (ish) for generating sound. Its very inefficient to generate low frequency sounds with a small thing (as you're fighting the mass of the air) or high frequency sounds with a large thing (as your fighting the mass of the thing).
So your dog things will need two hearing systems, and two noise generating systems, one for each purpose.
Low frequencies carry quite well through solid objects, so maybe they transmit their communications into the ground by impacting the ground, and hear with their feet. Think about a cowboy putting his ear to the ground to hear a stampede or touching the rails to check for trains. Or go the more traditional route and have a resonating chamber like elephants or whales, but this would need to be quite big and would use a lot of energy to use. It would probably want an opening, like a big nostril or something.
The high frequency organ could be small, but would need to be on or near the surface of the body, so that the high frequencies can escape. Think about a bat's face, or a cricket's legs. Maybe, just like crickets, they generate sound with friction on the surface of their bodies. The ears for high frequency should be big, as they need to collect as much energy as possible, and giving them a complex shape will help with detecting directionality (which I could talk about for as long again).
I should stop now, but I could carry on for as much again very happily.