Log in with your email/username and password at https://web.voxer.com/.
Chrome is the only browser that supports Voxer via the web. You will want to double check that you are on the latest version of Chrome for the most optimial experience.
You will need speakers and a microphone to hear and send voice messages.
Once you login, click on the permissions settings on your browser and give Voxer access to your microphone in order to send voice messages.