Since 1947, analog tape recording has become the dominant method of sound recording and has been widely used in music, cultural outreach, scientific research, and news gathering, producing a large number of important audio archives. However, due to the presence of adhesives, mildew often develops on the tape magnetic layer and edges, posing a serious threat to long-term preservation and sound quality. ATR-FTIR and SEM were used to characterize the chemical and physical properties of magnetic tape. Penicillium and Aspergillus were used to prepare samples with different coverage degrees of mildew, and the corresponding audio samples were analyzed using Praat. The relationships between spectral characteristics, acoustic parameters, and mildew coverage were analyzed. Additionally, surface morphology and roughness were observed using laser microscopy. The research reveal the mechanisms by which mildew affects sound quality and provide a basis for improved preservation strategies.