For OpenSim we recommend to use the latest Firestorm viewer, that you can download here:
Firestorm supports 3D meshes for objects or avatarsa web-on-prim as well as OpenSim specific features like varregions.
Be sure to enable voice in your viewer's preference settings and that you have also enabled voice on the current parcel, on the Audio tab of the About Land window.
If you use the Vivox or Freeswitch voice systems, you do not to do anything special.
If you use the Mumble voice system, you have to install a special voice plug-in on top of your viewer. It replaces the standard voice plug-in that is used by Vivox and Freeswitch. After that the viewer only supports Mumble voice. You can find more information about Mumble voice here:
Avoid using wireless and congested network connections. OpenSim starts to behave unreliable on network connections with packet loss, high network delay or varying network bandwidth. You can check these network parameters in the viewer by opening the Statistics Bar.
If others use the same network connection, be sure that enough bandwidth remains for your viewer. Otherwise you may experience slow rezzing or your viewer may even not work reliable.
Viewer Network Settings
For your viewer do not use network bandwidth settings that are set too high. Depending on the network connection and the number of visitors a reasonable maximum network bandwidth value is between 500 and 2000 kbps. This parameter can be set in the viewer's Preferences window, in the Network > Performance tab.
We recommend that you use the higher performance HTTP texture fetching. Enable the Use HTTP texture fetching check box on the same Preferences window tab.
Avatar Appearance Problems
If you experience avatar appearance problems, have a look at the article about Ruth & Cloud Avatars.
Please post in this forum your experiences with various viewers, to share it with our user's community. Don't forget to mention the viewer name, version number and under which operating system you use that viewer.