If you want to set some song, voice recording or ringtone saved on computer as new ringtone on mobile phone, you will need to firstly connect your phone to computer via USB, then copy the audio file from desktop or laptop to mobile phone, after that go to find the audio file using a file manager on the phone and set it as ringtone. Is there an easier way to do this? AirDroid is such a free app that can help us export ringtones from Android phone to computer or import ringtones to Android phones wirelessly. Check out details below.
Go to download the AirDroid app from Google Play or other Android market onto your mobile phone first. It is free to download and use.
Launch AirDroid app on your Android phone. Its Transfer tab opens automatically. Click AirDroid Web, you will see two addresses you can use to access AirDroid on the phone from the web interface. We will use its numeric address other than the alphabetic address, because it does not require a subscription via email account.

On your PC or Mac, launch your favorite web browser, could it be IE, Edge, Chrome, Safari, Firefox, or else. Access the above numeric address of AirDroid Web. You’ll then get the connection request on the phone. Accept the request on the phone. The AirDroid for Web opens in your desktop browser.
Click to open Ringtones in AirDroid Web interface, all your existing ringtones, including both system ringtones and custom ringtones, will be displayed. To upload music, recordings, ringtones from computer to Android phone, click the Upload file button at the top right corner, browse to the folder where the music or ringtone is saved on the computer and transfer it to your mobile phone wirelessly over Wi-Fi.

Now go to change the ringtone and set the new custom ringtone as the default ringtone on the phone. See this guide to change ringtone on Lenovo phone, just as an example. Setting new ringtones involves similar process across different Android mobile phones, such as Samsung, LG, HTC, Lenovo, Motorola, Huawei, Xiaomi, etc.
Import, export ringtones without Wi-Fi or internet
You can also enable Wi-Fi hotspot on the phone, then connect your computer to the hotspot, and import ringtones from computer to mobile phone, or export ringtones from phone to computer wirelessly. No internet or Wi-Fi network required.
You can refer to these steps to turn on personal hotspot on Xiaomi & Redmi phones, and this tutorial to enable Wi-Fi hotspot on Lenovo mobile, just to name a few.
Transfer ringtones from one phone to another
The same app and method can also help us share ringtones between Android mobile phones.