The mechanical response of bulk metallic glasses is ultimately controlled by interatomic forces between the randomly distributed atoms. The study of collective dynamics in amorphous systems, either liquid or glassy, opens a window to better understand this interaction. We present here results of recent IXS experiments performed at the ID16 beamline of the European Synchrotron Radiation Facility (ESRF) on a metallic glass. In particular, the high-frequency response of Pd77Si16.5Cu6.5 alloys, produced as a ribbon with an ultra-fast quenching rate (∼10^6 K/s) and as a bulk with a slow one (∼10^2 K/s), was determined. Previous experiments showed a discrepancy between the sound speed of metallic glasses measured by ultrasounds and by IXS. This was explained as a consequence of density fluctuations in the glass, although other explanations are possible. Here, the dispersion relation of the acoustic modes at room temperature and pressure are measured for both samples, allowing us to compute the sound speed and discuss the differences in the acoustic properties of both samples.