Q.The audio signal cannot be transmitted directly into the space. Why?

Audio-frequency signals are unsuitable for direct radiation into space for three practical reasons: antenna size, power/attenuation, and mutual interference between different transmitters — modulation onto a carrier solves all three.

1. Antenna size: efficient radiation of an electromagnetic wave requires an antenna of size comparable to (typically at least $\lambda/4$ of) the wavelength of the signal. Audio frequencies are very low (20 Hz to 20 kHz), so their free-space wavelengths ($\lambda=c/f$) range from tens to thousands of kilometres — building an antenna of that size is completely impractical.

2. Effective power radiated / range: the power radiated by an antenna varies strongly with frequency (roughly as a high power of frequency for a given antenna size); at audio frequencies, radiated power is negligible and the signal would be absorbed/attenuated within a very short distance, never reaching useful range.

3. Mixing/interference of multiple signals: since almost all audio signals from different sources (voices, music, etc.) occupy roughly the same low-frequency band, if many stations broadcast their audio signals directly (unmodulated) they would all overlap in the same narrow frequency range and could not be separated or individually tuned into by a receiver.

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