Learn English – Sound of a zip fastener

In standard English, 'th' is always a dental fricative, a non-stopping, continuous movement of air between the tip of the tongue right and the bottom of the top teeth. It is almost exactly the lisp sound.

It is a rare sound in the world's languages, so most language leaners of English have trouble pronouncing it and attempt to approximate it by the fricatives s/z or the stop t/d.

There are some non-standard varieties of English that use t/d naturally instead of the fricative, like for example Sylvester Stallone in Rocky, or any number of street-wise New Yorkers. So don't emulate this unless you want to sound very urban (and a bit out-of-date)

As with most phonetic things, learning it comes with practice and listening and more practice. For 'th' in particular, a little bit of your tongue sticks past the top teeth, so may feel weird (or even rude depending on your culture).

There are several networking terms that can apply here.

• Poor quality indicates that there is an issue with the recording or playback of the audio. It can refer to the quality of the microphone, the ambient noise in the microphone's location, the processing capability of the computer to which the microphone is connected, the microphone's attempt to alleviate feedback, the quality of the speakers at the receiving end, or the processing capability of the computer receiving the audio.
• Lag generally refers to the delay between transmission of the sound and reception of it. If the other person is listening to you through a set of speakers, you will sometimes hear an "echo" when your voice is played through the other's speakers.
• Latency refers to the ability of the network to continuously stream the audio. This sounds like the other person is "breaking up" because the network is losing pieces of the audio stream.