Data and VOICE VLANs really should NOT be “merged”. Data communications are a constant stream of bits, whereas Voice comes in chunks. Merging them will almost completely kill your voice call quality.

A better solutions would be to keep them on seperate VLANS (VLAN1 for data, and say VLAN 5 for voice – Cisco best practices states that a lower VLAN number for voice is better, but i digress)

Your current configuration of “Interface VLAN 1″ w/ 2 different VLAN ID’s on it is really unusual and seems to be causing a few of your issues. I would suggest creating a VLAN 5 (Name Voice) and put the Voice VLAN IP on “Interface VLAN 5″.

Once the data and voice are segregated, then go with Snappers suggestion of settin up trunking between devices and using EIGRP if needed (theyre both cisco, so why not?) Put BOTH VLANS on BOTH swithes, trunk between them, and enable a bit of QOS to prioritize VLAN 5 (Voice) traffic over the data traffic and you should see a marked difference

In summary: Data: VLAN 1 – 10.91.192.0/24, Voice: VLAN 5 – 172.20.1.0/24. Put Both VLANs on BOTH switches, then trunk between them – your 4500′s are more than capable of handling this config..

You can then connect the two switches via crossover using these non-vlan-capable ports, and then use a routing protocol (i.e. EIGRP) on each. If you’re not already running one, then add to each:
ip routing
router eigrp 1
network 10.0.0.0
network 192.168.10.0
no auto-summary

You should then be able to link the 2 networks “transparently” – this assumes you are running a level of IOS on each that supports basic routing.

If you DON’T have a version that supports it, then you can put static routing. on the voip one, add ip route 192.168.10.0 255.255.255.0 0.10.31.2; and on the data one add ip route 10.10.30.0 255.255.255.0 10.10.31.1.

If your network is actually more complex than this, you may have more work to do.