Open iTunes.
Open iTunes preferences. Click “iTunes” and select “Preferences” on a Mac or click “Edit” and select “Preferences" on a PC.
Open “Import Settings” and select the “MP3 Encoder” as the “Import Using” audio format.
Click the menu next to “Settings” and select “Custom.”
Select 48 KBPS as the bit rate by clicking the drop down menu next to “Stereo Bit Rate.”
Click “OK” to save the changed settings. Then click “OK” to exit of “Import Settings” and click “OK” again to exit the iTunes preferences menu.
Click “File,” “Add to Library” and select the MP3 from your computer that you would like to convert to 48 KBPS. This will import the MP3 into your iTunes library.
Click on the imported MP3, select “Advanced” and click “Create MP3 Version.” The MP3 will then be converted to 48 KBPS. When the conversion is complete, the 48 KBPS MP3 file will appear directly below the original 128 KBPS MP3 file. (See References 1).
Open Switch.
Click “Add Files” and select the MP3 you would like to convert from 128 KBPS to 48 KBPS.
Choose “.mp3” for the “Output Format” and then click “Encoder Options.” Choose a bit rate of 48 KBPS by clicking the drop-down menu next to “Bit Rate.”
Click “OK” to confirm your bit rate selection.
Choose the “Output Folder” for your converted MP3 by clicking “Browse” and selecting where you would like to save the file.
Click “Convert” and the MP3 will be converted from 128 KBPS to 48 KBPS. (See References 2).
Open dBpoweramp.
Click “Browse” and select the MP3 you would like to convert from 128 KBPS to 48 KBPS.
Choose “MP3” for the “Converting To” format and then drag the “estimated bit rate” slider to choose 48 KBPS as the bit rate. Drag the slider to the right to increase the bit rate or drag the slider to the left to decrease the bit rate. The currently selected bit rate will appear next to “estimated bit rate” and change accordingly as you move the slider.
Choose the “Output Location” for your converted MP3 by clicking "Folder" and selecting where you would like to save the file.
Click “Convert” and the MP3 will be converted from 128 KBPS to 48 KBPS. (See References 3).