Sometimes your sound may not match the video you send. A speaker can stop talking, but the speech keeps playing back, or a sound of an explosion in the game comes much later than it should. This normally can also be fixed on user-side.
First things first, simply make a local recording on your PC/Laptop by using your encoder "Record Video" option to check if the sound is not synced there too. 

If it is so, open your encoder's audio mixer or advanced audio properties and try adjusting audio sync offset. Some encoders allow only positive offset, while others allow negative offset too - try values like 200 and -200, and see which one is better in production. Increase or lower the value depending on the result.

It's also good to check your audio frequency settings - the most common is 44.1Khz, while some encoders use 48.0Khz by default. Try changing this value to see if it helps.

If sound offset happens only on your stream through Restream, but not on your local record or stream to any other platform, please contact Restream support asking them to check your logs. Make sure to attach the recorded file and a link to the stream on which the audio offset is happening for you.