Comment 3 for bug 1835974

Easiest method is to just add the Debian non-free armhf repo to raspbian and install the Debian binaries.

Proceed at your own risk. Works OK for here.

wget -q https://ftp-master.debian.org/keys/release-10.asc -O- | apt-key add -
echo "deb http://deb.debian.org/debian buster non-free" >> /etc/apt/sources.list
apt-get update
apt-get install libttspico0

or without adding the repo:
wget http://ftp.us.debian.org/debian/pool/non-free/s/svox/libttspico0_1.0+git20130326-9_armhf.deb
wget http://ftp.us.debian.org/debian/pool/non-free/s/svox/libttspico-utils_1.0+git20130326-9_armhf.deb
apt-get install -f ./libttspico0_1.0+git20130326-9_armhf.deb ./libttspico-utils_1.0+git20130326-9_armhf.deb