The vowel sound in "lock" is a short o sound, which is a long vowel sound. It is similar to the vowel sound in "lot" or "hot".
Here's how to tell the difference:
* Short vowel sounds are typically quick and sharp, like the "a" in "cat" or the "i" in "bit".
* Long vowel sounds are typically longer and more drawn out, like the "a" in "cake" or the "i" in "bike".
Since the "o" in "lock" sounds like the "o" in "hot", it's a long vowel sound.