For discount voiceovers, modifications are at cost.  Order here if you need a round of modifications for a discount voiceover.
For premium voiceovers, most include a reasonable number of modifications within a specified time window after my first delivery at no additional cost. An exceptional number of modifications, modifications requested after this time window, and new scripting that may increase word count may be at additional cost.
If you are an agency or designer working on behalf of a client: Make sure your client knows that modifications may be at cost, and build modification costs into your rate and budget.
Of course, if I made a rare mistake, I’ll correct it for free!