There may be several reasons for the problem that the sound of Xiaomi Mi 13 is suddenly louder and softer when using Kuwo Music. The following are some possible causes and solutions:

1. Music player setting problem: Make sure the volume setting of your Kuwo music player is stable and there is no automatic gain adjustment or volume balance setting. . You can go into Kuwo Music's settings options to see if these options are available and adjust them appropriately.

2. Automatic volume adjustment function: Some mobile phones have an automatic volume adjustment function, which may cause the sound to be louder and softer. You can go into the sound options in your phone's settings to see if auto-adjustment is enabled and try disabling it.

3. Hardware problems: In some cases, the sound may be caused by hardware problems of the mobile phone. This may involve damage or poor contact to the speaker or headphone jack. You can try using headphones or external speakers. If the problem persists, you may need to contact the phone manufacturer for repair or replacement.

In short, if you encounter the problem of loud and soft sounds, you can first try to adjust the volume settings and disable the automatic adjustment function. If the problem persists, it is recommended that you contact the mobile phone manufacturer for further assistance.