Speaker issue may include the phenomena below:

No sound from the speaker;
Speaker sound quality - Sound quality is abnormal;
Low volume issue;

1. Check whether the alert slider is in the "Ring" mode and adjust volume to a high level;
2. Make sure the speaker hole is clean, not blocked by foreign matters;
3. Restart the phone to check if the issue persists.

If actions above are not helping, please contact with OnePlus after-sale support.