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"A multicast comes into the phone from the cisco switch..... It's not going to matter that a voice vlan is setup right?"
Yes and no.
The data VLAN traffic, that containing the multicast flood, is still going to consume switch port bandwidth (as also noted by Justin). Such can delay Voice VLAN traffic.
Once the traffic gets to the phone, the "switch" portion of the phone should not send the data VLAN traffic to the "phone" portion, so the "phone" portion doesn't have to logically deal with that traffic. (The latter is one of the advantages of "mini trunk" [described by Justin].)
To really get the full advantage of the data and voice VLANs, you need to have dedicated ports for both. I.e. Phone and PCs on their own network ports. (NB: BTW, one large VoIP vendor, notes you can have twice as many phones on the same VLAN when you don't "mini trunk"/share the switch ports.)
BTW, you can mitigate some of the sharing the switch port's bandwidth by using egress QoS to give preference to the voice packets. This doesn't work as well as dedicated ports, for a couple of reasons. First, a switch's QoS may have a hardware FIFO queue before logical dequeuing occurs. Second, many VoIP phones internal switch function, isn't too efficient. (For example, we've seen VoIP phone be a major bottleneck to the PC connected to them.)
BTW, are you sure it was a multicast flood and not a broadcast flood? If the former, does your switch support IGMP snooping?
