Sample default volume

Is there any way to set a default volume for the sample ?

I.e., if I set the sample volume at half value, I would hear it at max volume only if I set the volume command to 0MFF

In the volume properties of the sample itself you can set its default volume.
Any other value:use the volume column.
The 0Mxx command is the channel volume (for the whole channel including all the other notes that are triggered)

The volume properties works in a way that any volume command is relative to that value.
For example, setting sample volume to -80db won’t produce any sound even with a channel command set at its maximum.

In fact, I would a behaviour that should replicate the sample default volume of the MOD / XM format.
That works in this way:

if I set the sample volume to 32/64, this always play at half volume until any volume command is triggered

You can use a macro for the volume and automate it exactly how you want. Not exactly what you asked for, but you can slide the macro half way down and sample will play at half the volume, you may then use a command to raise the volume, but you would need another command to take it back to half.

You are still playing instruments in the pattern of which samples are only a part of it.
In that case, The Bellow’s suggestion to assign a macro to the volume level of the instrument and then automate it in a track using a macro device will more or less do what you want, but on instrument basis, not sample basis, unless you meant instrument the whole time.