@crispyoz
Just an update :
To install svox I needed more space.
I followed the instructions (https://docs.onion.io/omega2-docs/boot-from-external-storage.html) using the SD card.
At the same time I installed a swap file on the same sd card ...with some confusion between /mnt/overlay/swap.page and
/mnt/mmcblk0p1/swap.page
With the /mnt/mmcblk0p1/swap.page in /etc/rc.local the swap is active after a reboot !
The install of svox is easy and now pico2wave is ready . It is a component of svox.
Thank you for your help, and congratulations for the people who wrote the documentation of Onion ! It is "top"
Robert