Hello Christicor,
Thanks for using Apple Support Communities.
I see you're finding that your Timer volume is low compared to the other sounds from your device.
Which iPhone model are you using? If you're using an iPhone X or later, there's a possibility that this is a result of the Attention Aware feature. If you're looking at your iPhone while Attention Aware is active, it lowers the volume of your alert sound, and this sounds like it might be consistent with what you're experiencing: About Attention Aware features on your iPhone X or iPad Pro. If you're using one of the above models and you want to turn this off, tap Settings > Face ID & Passcode, and then turn off Attention Aware Features.
Otherwise, there may be a few other factors to consider. How long has this been happening? Have you restarted your iPhone since the issue began? Restart your iPhone, iPad, or iPod touch.
Also, are you running iOS 12.1.2 under Settings > General > About? If not, an update is a great idea. Here's how: How to back up your iPhone, iPad, and iPod touch and Update your iPhone, iPad, or iPod touch.
I hope this helps get you started.
Cheers!