You can use the "Mixing King" software to add background music to a recording.

This software reprocesses music according to specific occasions and generates new styles of music. The mixing includes special effects processing such as rhythm speed, male and female voice change, background music mixing, sound fade-in and fade-out, volume adjustment, etc.

Supports direct saving of popular formats such as WAV/WMA/MP3.

Extended information:

Unlimited multi-format recording: can record sounds from many devices such as microphones and systems in real time, supports selective recording of multiple devices, and does not require temporary recording files and can be saved to many popular formats such as WAV/WMA/MP3 at one time.

Music remix recording function: allows you to select a song (audio or video), and then perform various special effects processing on it, such as maintaining the original song while processing the tempo faster or slower, or changing the male and female voices. Transformation processing.

Baidu Encyclopedia - Mixed Recording King