Cisco WAN :: 1841 QoS Set-up For Voice Between IPSEC VPN Connected Sites

I have 3 sites.  Each site has a Cisco 1841 as its WAN router with a 10Mb direct internet access circuit connected to Fa0/0.  The sites are then connected to each other via site-to-site IPSEC VPN.  (The LAN switches in use at each site are Cisco 3750 series) [code]

Now, Site A has already been set-up with VoIP telephony.   The plan is to extend this to the other 2 offices.Auto QoS has been set-up on the switches and data and voice VLANs created in the same way for each office.
 
how should/do we extend the QoS for the voice over the WAN to ensure voice quality remains for site to site calls.  And what special considerations do we have to make for it being IPSEC VPN connectivity between the sites?  The actual IP telephony system itself is being set-up by a 3rd party and not a lot of information on their requirements has been forthcoming so far – essentially all we have really been told is that they would like us to “reserve” a certain amount of bandwidth for the voice traffic between each site.