Yeah you need to subscribe to HD PVR service (RM26.50/month) to watch HD channels corresponding to the package. If you've subscribed only PVR service, then you need to upgrade it.

For example to enjoy Astro Supersport HD, you need to subscribe both Sports pack and HD-PVR Service.

Dolby 5.1 on Astro is only available on selected HD channels only. SD channels are only Stereo/Mono.
The surround sound are great, especially from movies channels like HBO and Fox Movies. Remember to change Digital Audio Settings from your Astro box from Stereo to Dolby Digital 5.1, but do note that this will result your RCA output from Astro box or TV (that do not support Dolby audio passthrough) to have no sound when the channel is Dolby 5.1 (e.g. HD channels). If this happens, you got to switch back to Stereo, and you shall switch to Dolby 5.1 only whenever you want to enjoy surround sound.

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5.1 audio for TV3 is not yet available on Astro, now it is only available on digital TV platform.
TV3 (HD) now uses HE-AAC codec for 5.1 audio, no longer Dolby audio codec. If the audio is 5.1 but it's not AC3 or E-AC3 or DTS, using Optical audio (TOSLINK) will not output as 5.1 surround audio, even if your setting has been set as "Bitstream". The best option is use HDMI (ARC) method to output 5.1 audio instead of using optical cable. For nowadays, optical cable is used only if your home theatre system (especially older ones) do not have HDMI input, to enjoy surround sound. You can Google more about this, if you're interested.