ADSP-BF518 MP3 codec is not supporting for all frequency of audio signal. Request to share if there is any reference code which can help to process audio signal working in Any frequency
The software modules which are available currently are listed in the below link:
Also please note that the Blackfin MP3 decoder supports the following frequencies:
• All sampling frequencies as specified by  (ISO/IEC 11172-3, a.k.a. ‘MPEG1’, 32, 44.1,and 48kHz) and  (ISO/IEC 13818-3, a.k.a. ‘MPEG2’, 16kHz, 22.05kHz, and 24kHz)
• Support for sampling frequencies of 8kHz, 11.025kHz, and 12 kHz (for an unofficial standard, widely known as ‘MPEG2.5 MP3’)
You can find the more information in the "KT-7_MP3_Decoder_BF_Product_Reference" manual which available in the MP3 decoder installation path.
<installation drive>:\Analog Devices\SoftwareModules\MP3_Decoder-BF-Rel4.0.0\Documents