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If you are interested in setting QoS for voice, then you should apply qos on physical interface of the spokes and classify traffic via dscp marking. QoS pre-classify would be required on the tunnel.
BTW, if you're using ToS for physical egress, most DMVPN, I believe, copies original packet's ToS to GRE's ToS, so pre-classify isn't necessary. Pre-classify is necessary if your physical egress wants to examine other than ToS. An example, would be using FQ within an egress class which needs to "see" original src/dest to identify different flows.
