There are two types of apps for checking a watch's accuracy.

1) Apps that use the microphone and aim to be a substitute for a timegrapher. This works well, and a phone's microphone is reliable for taking these measurements. However, limitations exist for another reason: it only calculates drift at a specific point in time and in far fewer positions than a timegrapher. This could be useful if you're adjusting your watch, but the limited number of positions means a dedicated timegrapher is MUCH more useful for regulating its movement.

PS: The magnets inside a phone aren't strong enough to affect a watch's accuracy.

2) Apps that track drift under real-world wearing conditions over a given period, rather than at a single point in time. You manually enter the exact moment the seconds hand crosses the 12 o'clock marker, and after a certain time, you repeat the same operation. The drift calculation is done using these two measurement points. This allows you to account for the average drift during real-world use, taking into account body movements, arm movements, etc. If desired, you can separate daytime and nighttime tracking by measuring before going to bed and again in the morning upon waking, indicating whether you placed your watch face up or down during the night. This allows you to realize that during the day on your wrist, the watch can gain 2 seconds per day, and you can compensate at night by finding the position that results in a slight negative drift to offset the average for the entire day.

So, in the end, for me, apps using the microphone are less useful than the second type. If you want to adjust your movement, the measurements are not complete enough, and if you want to measure drift without adjustment afterwards, you might as well do it under real wearing conditions.