If the system hosting the Ventrilo server has a static IP address then the procedure is rather simple.
Firstly, do not add the voice.domain.com sub-domain to the DreamHost hosting system via Domains -> Manage Domains -> Add New Domain / Sub-Domain. If you have already done this, remove the sub-domain and wait for the changes to make their way through the system, this can take a little while.
Now, to 're-direct' the sub-domain to an external IP address you simply need to create an A-Record in the DNS records of the parent domain.
In the Panel go to Domains -> Manage Domains, click the small [DNS] link next to the parent domain and in the section titled 'Add a DNS record to domain.com' add the following record;
Click the Add Record Now! button and wait for the changes to take affect, once again, these changes may take a little while to make their way through the system.
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