Low-temperature heat capacity is systematically investigated in various glassy and crystalline polymorphs of a wide range of metallic glasses. We reveal that the boson heat capacity peak beyond the Debye level arises from both excess phonon scattering and background electronic excitation, and the two contributions are strongly coupled and also material-dependent. It is interesting to observe that the boson heat capacity peaks obey an inversely linear correlation between their heights and characteristic positions, which is mainly dominated by phonic anomalies. This indicates a universal structural softening among the studied glasses when the boson peak occurs. We further suggest a possibility that the linear evolution of the fast boson peaks can probe into the slow structural softening across the glass transition, and the two dynamic processes are controlled by the short-time shear modulus associated with local soft regions in fragile glasses. Published by AIP Publishing.